Muted submission response inconsistent with system notices and translations

[timkinnane]

v0.35.0

There's 3 problems with the current behaviour when a user who is muted submits a message:

1. The message appears them to be posted to the room, then they get a message that they've been muted. Its unclear if the submitted messages was actually posted though, but other users don't see it. This puts the muted users view of the room out of sync with the real room history. We've hacked a fix to this for a specific context using onClientBeforeSendMessage, but it needs to be brought inline with the global muting behaviour whatever that turns out to be, in regard to the following:
2. The error response comes from Rocketbot. This is inconsistent with all other system/permissions notices and the only such usage of Rocketbot. In some instances rocketbot is disabled and it doesn't make sense to surface it for one message type.
3. The message coming form the bot cannot use the same approach for translations as other system wide messages. Should use a translatable key string for consistent localisation.

I think the behaviour of muted user submitting a message should either block submission, with a standard system error alert OR the message box should be disabled or removed entirely with a view helper based on user-is-muted, showing the "You have been muted..." description instead as a placeholder.

[timkinnane]

Resolved. Related discussion on #10456