Notification spam for related actions

[dgw]

In an ideal world, this plugin would output only one message for situations like these examples:

• new issue/PR opened with labels/milestone/assignee (others?) already set
  • target output: a single unified presentation of all details pertaining to the new issue/PR, on one line (though it may use max_messages to have Sopel split a long line)
• milestone removed, then another added
  • target output: a single "updated the milestone" message
• multiple labels added/removed in quick succession
  • target output: a single message listing the labels added and removed

I think these fairly common occurrences can be built with future expandability in mind. Coalescing notifications like this should happen only for events on the same issue/PR from the same user.

[dgw]

New thought: This seems like it'll almost deserve its own release due to how much of the webhook stuff will need to be rewritten.