This commits adds the steps for configuring message retention policy
for an organization and for individual streams in message retention
policy docs.
Fixes#15495.
This commits adds restriction on admins to set message retention policy.
We now only allow only organization owners to set message retention
policy.
Dropdown for changing retention policy is disabled in UI for admins also.
We've completed the production performance testing that had been
blocking us from advertising this feature, so we're declaring the
feature implemented by documenting it.
As of this commit, you need to edit fields using `manage.py shell` to
configure it and manually configure the cron job, but those should be
addressed in the next few days.
Fixes#106.