Allow for separate moderator's notifications room

Nobody has claimed this yet.

Looked through the open issues list, and searched the PRs (both closed&open), found nothing:

# Problem

I have the mjolnir admin channel on high alert in my matrix client, because us moderators can't miss a reported message for long.

Sadly, every list update, every automatic maintenance/restart/… message is also inserted into that room. Making that room 95% noise, 5% really really important and easy to miss messages.

That seems, at least to me, terrible UI, on the level of badness that makes nuclear power plants catch fire and airplanes crash.

# Proposed approach

Allow to have a separate channel that *only* receives messages about reported messages.

There's two incarnations that could take:

1. notification only: no way to interact with Mjolnir, especially not to block/ban/redact. Easiest, I guess. Moderators need to switch channel to execute on reports, but it allows them to mute the noisy notifications of the main channel.
2. full interaction: anything you could do with Mjolnir. Basically, this channel would be a functional copy of the main channel, just minus any message that would qualify as "log". This might lead to user confusion.
3. moderation-interaction only. Gets the report, and can run only the commands that are related to user management, but not room or server management.

Start by tracing how Mjolnir currently sends messages to its admin channel and how reported messages are handled. The issue leaves three interaction models open, so done requires choosing and implementing a separate report-notification channel with a clear command scope, then verifying that routine maintenance and list updates do not appear there.

Written by the indexing model from the issue text.