This commit adds variousMessageBus.last_ids to serializer payloads
for chat channels and the chat view (for chat live pane) so
we can use those IDs when subscribing to MessageBus channels
from chat.
This allows us to ensure that any messages created between the
server being hit and the UI loaded and subscribing end up being
delivered to the client, rather than just silently dropped.
This commit also fixes an issue where we were subscribing to
the new-messages and new-mentions MessageBus channels multiple
times when following/unfollowing a channel multiple times.
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.
This is a followup of the previous refactor where we created two new
models to handle all the dedicated logic that was present in the
`ChatChannel` model.
For the sake of consistency, `DMChannel` has been renamed to
`DirectMessageChannel` and the previous `DirectMessageChannel` model is
now named `DirectMessage`. This should help reasoning about direct
messages.
