Add purge command

[dawnofmidnight]

(part of the Carl ⇒ Ferris migration) (pending discussion from mods, wait for approval to implement)

We'd like some way to mass-delete messages in a particular channel or from a particular channel. For users, I think the usage is rather simple, where the limit here can either be specified in a time specification (i.e. "last ten minutes" as 10m) or a number of messages (bounded to a specific number as both a check and to avoid being interpreted as a channel ID if that's omitted):

• ?purge user <user> <limit>: delete all messages from a particular user up to the limit

I see two possible designs for channel deletions:

• ?purge channel [channel] <limit>: delete all messages in the channel up to the limit. The channel should be optional.

OR

• ?purge since <message link>: delete all messages since that message. ensures that the boundary is clear.
• ?purge between <message link> <message link>: delete all messages between these two boundaries after checking that they're both in the same channel.

This pair isn't necessarily mutually exclusive. However, I do much prefer the message link approach since it's a lot harder to use it incorrectly and delete messages that should've been kept.

[troiganto]

The message link approach sounds interesting. I see no reason why we couldn't have both approaches, they seem difficult to mix up, especially given the abilities of slash commands.