streams: Render and store the stream description from the backend.

This commit does the following three things:
    1. Update stream model to accomodate rendered description.
    2. Render and save the stream rendered description on update.
    3. Render and save stream descriptions on creation.

Further, the stream's rendered description is also sent whenever the
stream's description is being sent.

This is preparatory work for eliminating the use of the
non-authoritative marked.js markdown parser for stream descriptions.

zerver/lib/bulk_create.py

@@ -4,6 +4,7 @@ from zerver.lib.initial_password import initial_password
 from zerver.models import Realm, Stream, UserProfile, Huddle, \
     Subscription, Recipient, Client, RealmAuditLog, get_huddle_hash
 from zerver.lib.create_user import create_user_profile
+from zerver.lib.bugdown import convert as bugdown_convert
 
 def bulk_create_users(realm: Realm,
                       users_raw: Set[Tuple[str, str, str, bool]],
@@ -75,6 +76,7 @@ def bulk_create_streams(realm: Realm,
                     realm=realm,
                     name=name,
                     description=options["description"],
+                    rendered_description=bugdown_convert(options["description"]),
                     invite_only=options.get("invite_only", False),
                     is_announcement_only=options.get("is_announcement_only", False),
                     history_public_to_subscribers=options["history_public_to_subscribers"],