Pre-fetch data from the DB and hand to markdown thread

We want to avoid opening a DB connection in the markdown thread
as its DB connection might live for a long time

(imported from commit 7700b2ca793ee5e9add7f071b92f22a4bf576b3d)

zerver/lib/mention.py

@@ -9,16 +9,5 @@ find_mentions = r'(?<![^\s\'\"\(,:<])@(?:\*\*([^\*]+)\*\*|(\w+))'
 
 wildcards = ['all', 'everyone']
 
-def find_user_for_mention(mention, realm):
-    if mention in wildcards:
-        return (True, None)
-
-    try:
-        user = zerver.models.UserProfile.objects.filter(
-                Q(full_name__iexact=mention) | Q(short_name__iexact=mention),
-                is_active=True,
-                realm=realm).order_by("id")[0]
-    except IndexError:
-        user = None
-
-    return (False, user)
+def user_mention_matches_wildcard(mention):
+    return mention in wildcards