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 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.