Notify user when ratelimiting is activated/deactivated

[Phil Connell]

The standard ratelimit plugin logs when it's dropping messages (e.g. "Ratelimiting msgs to j@i.d"), but gives no indication to affected senders/recipients.

This has proved irritating for endroid_remote users: if they (perhaps accidentally) 'edr cat' a lot of output, some is likely to be mysteriously dropped from their point of view.

This bug tracks sending some notification of ratelimiting beginning/ending to senders/recipients of messages. As noted above, it's particularly desirable for endroid remote.