messages: Add support for admins deleting messages.

This makes it possible for Zulip administrators to delete messages.
This is primarily intended for use in deleting early test messages,
but it can solve other problems as well.

Later we'll want to play with the permissions model for this, but for
now, the goal is just to integrate the feature.

Note that it saves the deleted messages for some time using the same
approach as Zulip's message retention policy feature.

Fixes #135.

zerver/lib/actions.py

@@ -28,6 +28,7 @@ from zerver.lib.message import (
     render_markdown,
 )
 from zerver.lib.realm_icon import realm_icon_url
+from zerver.lib.retention import move_message_to_archive
 from zerver.models import Realm, RealmEmoji, Stream, UserProfile, UserActivity, \
     RealmDomain, \
     Subscription, Recipient, Message, Attachment, UserMessage, RealmAuditLog, \
@@ -2821,6 +2822,19 @@ def do_update_message(user_profile, message, subject, propagate_mode, content, r
     send_event(event, list(map(user_info, ums)))
     return len(changed_messages)
 
+
+def do_delete_message(user_profile, message):
+    # type: (UserProfile, Message) -> None
+    event = {
+        'type': 'delete_message',
+        'sender': user_profile.email,
+        'message_id': message.id}  # type: Dict[str, Any]
+    ums = [{'id': um.user_profile_id} for um in
+           UserMessage.objects.filter(message=message.id)]
+    move_message_to_archive(message.id)
+    send_event(event, ums)
+
+
 def encode_email_address(stream):
     # type: (Stream) -> Text
     return encode_email_address_helper(stream.name, stream.email_token)