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.