This commit automatically ensures that category channels
have slugs when they are created or updated based on the
channel name, category name, or existing slug. The behaviour
has been copied from the Category model.
We also include a backfill here with a simplified version
of Slug.for with deduplication to fill the slugs for already
created Category chat channels.
The channel slug is also now used for chat notifications,
and for the UI and navigation for chat. `slugifyChannel`
is still used, but now does the following fallback:
* Uses channel.slug if it is present
* Uses channel.escapedTitle if it is present
* Uses channel.title if it is present
In future we may want to remove this altogether
and always rely on the slug being present, but this
is currently not possible because we are not generating
slugs for DM channels at this point.
