Handle users editing/deleting messages with conversions

[Phanabani]

When a user edits a conversion, Sandpiper should pick up on this and edit her reply with the new conversion. Likewise, if a user deletes their message with a conversion, she should delete her reply. The deletion could also be a config field.

[Phanabani]

I started work on this a while ago and implemented it with a per-channel cache to map messages to Sandpiper replies. This is good because she doesn't need to fetch messages to find her reply, but the downside is that the cache is in-memory only, so she'll lose it on restarts, which I think defeats the purpose.

I think then I will use the fetch method. When a message is edited/deleted, fetch messages forward in time until it finds her reply (with a limit). This operation may be ambiguous if two people did a conversion at the same time, so I think we should use #56 to definitively get the correct reply.