Implement separate bot users per service

[justinbot]

Introduces the ability for Hookshot to present itself as multiple different bots, each of which can handle one or more specific services. This is a pretty fundamental change, and as a result touches almost everywhere.

It should be relatively reviewable by commit, but will require significant testing of different scenarios to make sure things work correctly.

Implementation

Fundamentally, this is breaking an assumption throughout the codebase that there is one global bot. The implications are:

• Before interacting with a room, we need to find a bot which is in the room.
  • But also, multiple bots could be in the same room.
• When doing so in the context of a particular service (e.g. sending a feed update), we need to prioritize the service bot over the default bot.

Connections

However, this does not mean connections are tied to a specific bot user.
We need to choose the right bot for a connection, but this is done at runtime based on which bots are in the room (again prioritizing service bots).

Fixes #436

[justinbot]

@Half-Shot (I can't reply to this comment directly for some reason)

This should probably be a dedicated handover function. Our connections are slightly stateful and it's better to not recreate them v.s. updating them, if we can avoid it.

It doesn't seem like it can be avoided, since "handing over" a connection really means instantiating it with a different intent in the opts.
If there are specific statefulness concerns I would say let's try and address those, since anyway connections need to be recreated from state events when e.g. Hookshot is restarted.

[tadzik]

I'm happy with this – I'm still curious about #573 (comment) though (defering to Half-Shot).