There are many situations that may cause users to lose permission to
send messages in a chat channel. Until now we have relied on security
checks in `Chat::ChatChannelFetcher` to remove channels which the
user may have a `UserChatChannelMembership` record for but which
they do not have access to.
This commit takes a more proactive approach. Now any of these following
`DiscourseEvent` triggers may cause `UserChatChannelMembership`
records to be deleted:
* `category_updated` - Permissions of the category changed
(i.e. CategoryGroup records changed)
* `user_removed_from_group` - Means the user may not be able to access the
channel based on `GroupUser` or also `chat_allowed_groups`
* `site_setting_changed` - The `chat_allowed_groups` was updated, some
users may no longer be in groups that can access chat.
* `group_destroyed` - Means the user may not be able to access the
channel based on `GroupUser` or also `chat_allowed_groups`
All of these are handled in a distinct service run in a background
job. Users removed are logged via `StaffActionLog` and then we
publish messages on a per-channel basis to users who had their
memberships deleted.
When the user has a channel they are kicked from open, we show
a dialog saying "You no longer have access to this channel".
When they click OK we redirect them either:
* To their first other public channel, if they have any followed
* The chat browse page if they don't
This is to save on tons of requests from kicked out users getting messages
from other channels.
When the user does not have the kicked channel open, we can just
silently yoink it out of their sidebar and turn off subscriptions.
