Add ability to (re)move off-topic messages with bot

[dector]

Use case: somebody is posting off-topic message to chat.

Now:

User A:
    Hi! How do you like % NON KOTLIN-RELATED STUFF %?
User B:
    >>> User A:
    >>> Hi! How do you like % NON KOTLIN-RELATED STUFF %?
    Oh, not again!
User C:
    >>> User A:
    >>> Hi! How do you like % NON KOTLIN-RELATED STUFF %?
    GFY!
User D:
    >>> User A:
    >>> Hi! How do you like % NON KOTLIN-RELATED STUFF %?
    Belongs to @pofftop !

Better option

User A:
    Hi! How do you like % NON KOTLIN-RELATED STUFF %?
Moderator:
    >>> User A:
    >>> Hi! How do you like % NON KOTLIN-RELATED STUFF %?
    !mod offtopic

Then bot (re)moves offtopic message to special chat and removes moderator reaction.

Note: !mod offtopic is just a command sample, can be discussed.

[r4zzz4k]

Just my two cents: it sounds reasonable to allow same / similar command for usual members, but react to them after reaching some kind of threshold. We'll probably get to it at some point at Heapy/kotbot.