This commit adds a management command that will run regularly
as a cron job to send zulip updates to realms based on their
current and latest zulip_update_announcements_level.
For realms with:
* level = None: Send a group DM to admins notifying them about
this new feature & suggestion to set the stream accordingly.
* level = 0:
* If stream is still not configured, wait for a week
before setting their level to latest level. They will
miss updates until their configure the stream.
* If stream is configured, send updates.
* level > 0: Send one message/update per level & increase
the level by 1 till the latest level.
Fixes#28604.
Prakhar Pratyush