Add widget.py to enable widgets on the server side.

The only slash command implemented in this initial
version is an extremely crippled version of a
"/stats" slash command that reports that you are
running 1 server.

tools/test-backend

@@ -93,6 +93,8 @@ not_yet_fully_covered = {
     'zerver/lib/test_fixtures.py',
     'zerver/lib/test_runner.py',
     'zerver/lib/api_test_helpers.py',
+    # Experimenatal
+    'zerver/lib/widget.py',
     # Webhook integrations with incomplete coverage
     'zerver/webhooks/basecamp/view.py',
     'zerver/webhooks/beanstalk/view.py',

zerver/lib/actions.py

@@ -88,6 +88,8 @@ from zerver.models import Realm, RealmEmoji, Stream, UserProfile, UserActivity,
 from zerver.lib.alert_words import alert_words_in_realm
 from zerver.lib.avatar import avatar_url, avatar_url_from_dict
 from zerver.lib.stream_recipient import StreamRecipientMap
+from zerver.lib.widget import do_widget_post_save_actions, \
+    get_fixed_content_for_widget
 
 from django.db import transaction, IntegrityError, connection
 from django.db.models import F, Q, Max, Sum
@@ -1137,6 +1139,10 @@ def do_send_messages(messages_maybe_none: Sequence[Optional[MutableMapping[str,
         message['sender_queue_id'] = message.get('sender_queue_id', None)
         message['realm'] = message.get('realm', message['message'].sender.realm)
 
+        # Some widgets just overwrite content.
+        fixed_content = get_fixed_content_for_widget(message['message'].content)
+        message['message'].content = fixed_content
+
         mention_data = bugdown.MentionData(
             realm_id=message['realm'].id,
             content=message['message'].content,
@@ -1238,6 +1244,9 @@ def do_send_messages(messages_maybe_none: Sequence[Optional[MutableMapping[str,
             if Message.content_has_attachment(message['message'].content):
                 do_claim_attachments(message['message'])
 
+        for message in messages:
+            do_widget_post_save_actions(message)
+
     for message in messages:
         # Deliver events to the real-time push system, as well as
         # enqueuing any additional processing triggered by the message.

zerver/lib/widget.py

@@ -0,0 +1,19 @@
+from typing import MutableMapping, Any
+from django.conf import settings
+
+def get_fixed_content_for_widget(content: str) -> str:
+    if not settings.ALLOW_SUB_MESSAGES:
+        return content
+
+    if content == '/stats':
+        return 'We are running **1 server**.'
+
+    return content
+
+def do_widget_post_save_actions(message: MutableMapping[str, Any]) -> None:
+    '''
+    This is experimental code that only works with the
+    webapp for now.
+    '''
+    if not settings.ALLOW_SUB_MESSAGES:
+        return