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.

tools/linter_lib/custom_check.py

@@ -574,6 +574,7 @@ def build_custom_checkers(by_lang):
              'zerver/migrations/0032_verify_all_medium_avatar_images.py',
              'zerver/migrations/0060_move_avatars_to_be_uid_based.py',
              'zerver/migrations/0104_fix_unreads.py',
+             'zerver/migrations/0206_stream_rendered_description.py',
              'pgroonga/migrations/0002_html_escape_subject.py',
          ]),
          'description': "Don't import models or other code in migrations; see docs/subsystems/schema-migrations.md",