This commit adds the messages_count column for ChatChannel messages,
which is the number of not-deleted messages in the channel.
This is not updated every time a message is created or deleted in a
channel, so it should not be displayed in the UI.
It is updated eventually via Jobs::ChatPeriodicalUpdates, which
will have additional functions in future after being introduced
here.
Also update these counts for existing channels in a post migration.
The settings tab of each category channel should now present the option to allow or disallow channel wide mentions: @here and @all.
When disallowed, using these mentions in the channel should have no effect.
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.
