Allow the bot to respond to reports with a reaction

[GregSutcliffe]

In some circumstances it is nice to have some indication that the bot saw your report via reactions - especially where there is heavy use of notice_emoji which suppresses the text acknowledgement.

This PR adds an ack_emoji, and allows for it to be disbled in the config. Best paired with #62 which would allow you to either disabled all text and just use reactions, or disable reactions and just use text, or a mix of both.

[haecker-felix]

I thought a bit about it, I don't think we should introduce another emoji. I think your described behaviour also can be achived by using the notice_emoji. I'm not 100% sure yet how, but one possibility would that every news message also gets the notice_emoji automatically added.

@thibaultamartin what do you think?

[GregSutcliffe]

My use-case was fairly niche, and I wanted it to be super-obvious that the bot had noticed, without cluttering the chat. Think of using this to run a meeting, where there's lots of chat, but the bot should store the official records - you don't want it replying with "OK, I'll store that" many many times in the meeting.

However, I guess the bot could add a second notice emoji - my concern would only be that it would be hard to see - not all clients report the total / userIDs for reactions. But I accept that use-case might be outside what you want the bot to handle. Very interested to hear what @thibaultamartin thinks too!