alert words: Refactor alert word detection.

The bugdown parser no longer has a concept of which users need which
alert words, since it can't really do anything actionable with that info
from a rendering standpoint.

Instead, our calling code passes in a set of search words to the parser.
The parser returns the list of words it finds in the message.
Then the model method builds up the list of user ids that should be
flagged as having alert words in the message.

This refactoring is a little more involved than I'd like, but there are
still some circular dependency issues with rendering code, so I need to
pass in the rather complicated realm_alert_words data structure all the way
from the action through the model to the renderer.

This change shouldn't change the overall behavior of the system, except
that it does remove some duplicate regex checks that were occurring when
multiple users may have had the same alert word.

zerver/lib/actions.py

@@ -25,6 +25,7 @@ from zerver.models import Realm, RealmEmoji, Stream, UserProfile, UserActivity,
     ScheduledJob, realm_filters_for_domain, get_owned_bot_dicts, \
     get_old_unclaimed_attachments, get_cross_realm_users
 
+from zerver.lib.alert_words import alert_words_in_realm
 from zerver.lib.avatar import get_avatar_url, avatar_url
 
 from django.db import transaction, IntegrityError
@@ -629,8 +630,12 @@ def do_send_message(message, rendered_content = None, no_log = False, stream = N
 
 def render_incoming_message(message, content):
     # type: (Message, text_type) -> text_type
+    realm_alert_words = alert_words_in_realm(message.get_realm())
     try:
-        rendered_content = message.render_markdown(content)
+        rendered_content = message.render_markdown(
+            content=content,
+            realm_alert_words=realm_alert_words,
+        )
     except BugdownRenderingException:
         raise JsonableError(_('Unable to render message'))
     return rendered_content