management: Add command to send a custom email to users.

This allows us to email sets of users on a server with a nicely
formatted email similar to our onboarding emails, built off of a
Markdown template.

The code was based on send_password_reset_email, but it doesn't
replace that use case, since one cannot include special values like
password reset tokens in these emails.

zerver/management/commands/send_custom_email.py

@@ -0,0 +1,103 @@
+import hashlib
+import shutil
+import subprocess
+
+from argparse import ArgumentParser
+from typing import Any, Dict, List
+
+from zerver.lib.management import CommandError, ZulipBaseCommand
+from zerver.lib.send_email import FromAddress, send_email
+from zerver.models import UserProfile
+from zerver.templatetags.app_filters import render_markdown_path
+
+def send_custom_email(users: List[UserProfile], options: Dict[str, Any]) -> None:
+    """
+    Can be used directly with from a management shell with
+    send_custom_email(user_profile_list, dict(
+        markdown_template_path="/path/to/markdown/file.md",
+        subject="Email Subject",
+        from_name="Sender Name")
+    )
+    """
+
+    with open(options["markdown_template_path"], "r") as f:
+        email_template_hash = hashlib.sha256(f.read().encode('utf-8')).hexdigest()[0:32]
+    email_id = "zerver/emails/custom_email_%s" % (email_template_hash,)
+    markdown_email_base_template_path = "templates/zerver/emails/custom_email_base.pre.html"
+    html_source_template_path = "templates/%s.source.html" % (email_id,)
+    plain_text_template_path = "templates/%s.txt" % (email_id,)
+    subject_path = "templates/%s.subject.txt" % (email_id,)
+
+    # First, we render the markdown input file just like our
+    # user-facing docs with render_markdown_path.
+    shutil.copyfile(options['markdown_template_path'], plain_text_template_path)
+    rendered_input = render_markdown_path(plain_text_template_path.replace("templates/", ""))
+
+    # And then extend it with our standard email headers.
+    with open(html_source_template_path, "w") as f:
+        with open(markdown_email_base_template_path, "r") as base_template:
+            # Note that we're doing a hacky non-Jinja2 substitution here;
+            # we do this because the normal render_markdown_path ordering
+            # doesn't commute properly with inline-email-css.
+            f.write(base_template.read().replace('{{ rendered_input }}',
+                                                 rendered_input))
+
+    with open(subject_path, "w") as f:
+        f.write(options["subject"])
+
+    # Then, we compile the email template using inline-email-css to
+    # add our standard styling to the paragraph tags (etc.).
+    #
+    # TODO: Ideally, we'd just refactor inline-email-css to
+    # compile this one template, not all of them.
+    subprocess.check_call(["./scripts/setup/inline-email-css"])
+
+    # Finally, we send the actual emails.
+    for user_profile in users:
+        context = {
+            'realm_uri': user_profile.realm.uri,
+            'realm_name': user_profile.realm.name,
+        }
+        send_email(email_id, to_user_ids=[user_profile.id],
+                   from_address=FromAddress.SUPPORT,
+                   from_name=options["from_name"], context=context)
+
+class Command(ZulipBaseCommand):
+    help = """Send email to specified email address."""
+
+    def add_arguments(self, parser: ArgumentParser) -> None:
+        parser.add_argument('--entire-server', action="store_true", default=False,
+                            help="Send to every user on the server. ")
+        parser.add_argument('--markdown-template-path', '--path',
+                            dest='markdown_template_path',
+                            required=True,
+                            type=str,
+                            help='Path to a markdown-format body for the email')
+        parser.add_argument('--subject',
+                            required=True,
+                            type=str,
+                            help='Subject line for the email')
+        parser.add_argument('--from-name',
+                            required=True,
+                            type=str,
+                            help='From line for the email')
+
+        self.add_user_list_args(parser,
+                                help="Email addresses of user(s) to send emails to.",
+                                all_users_help="Send to every user on the realm.")
+        self.add_realm_args(parser)
+
+    def handle(self, *args: Any, **options: str) -> None:
+        if options["entire_server"]:
+            users = UserProfile.objects.filter(is_active=True, is_bot=False,
+                                               is_mirror_dummy=False)
+        else:
+            realm = self.get_realm(options)
+            try:
+                users = self.get_users(options, realm, is_bot=False)
+            except CommandError as error:
+                if str(error) == "You have to pass either -u/--users or -a/--all-users.":
+                    raise CommandError("You have to pass -u/--users or -a/--all-users or --entire-server.")
+                raise error
+
+        send_custom_email(users, options)