Add starboard support

[dawnofmidnight]

(part of the Carl ⇒ Ferris migration)

Implement starboard support in Ferris. When a message gets a certain number of ⭐ reactions, it should be sent to a specific channel (specified by env var with ID). This should support the following commands:

• (mod only) ?starboard setlimit <n>: sets the required number of stars for the starboard to <n>

Open questions:

• Do we want the emoji to be configurable?
• Do we want to have user stats like Carl does?
• Should messages which go below the limit (7 ⇒ 6 if 7 is the limit) be de-starboarded? I don't think so, because that just gives people extra notifications if it's removed and then added a few minutes later.
• How do we prevent a message showing up in the starboard twice because of reactions being removed and re-added? Possible solutions:
  • Maintain some database table which keeps track of which messages have been starboarded. This would be necessary if we want user stats anyway.
  • If we don't, we could have Ferris just react to the same message with a ✔️ or some other emoji. If it detects that from itself on that message, then it'll ignore it.
• How do we deal with messages that have been starboarded by Carl previously?

[troiganto]

One thing that's missing is the ability to exclude certain channels from starboarding. That's not super relevant for RPLCS, but it would be e.g. to ROUS if they want to use Ferris as well.

[david-d-h]

#offtopic was just blacklisted completely from the starboard, to my knowledge solely due to the memes-club subthread. I suggested the possibility of still having a starboard-like experience in #memes-club by pinning messages that would normally be on starboard to the #memes-club thread instead.

This raises a bunch of questions which don't really belong in this issue, a new one would have to be created if this is considered for implementation.

Another approach is to have multiple starboard channels, and the possibility to redirect starred messages from specific channels to such a starboard channel.

The latter is probably the more sound option, which is why I'm commenting on this issue first before creating a new one. I'd be happy to hear your feedback.

I'm also putting myself out there as I'd be willing to implement the former suggestion. (It might take me some time)