Conversations UX issues

[cyphar]

There are two major problems with conversations:

• Right now if there is a conversation in a room, all users will be put into a conversation. This is because conversation state is a per-room property rather than being associated with a user (or username).
• The configured bot name is shown in the button which doesn't match the author, and thus makes the whole experience less smooth (unless you rename your bots and make it impossible to debug). This may be less painful if we add highlight which messages triggered which bots (or would trigger?) #65 but I feel like a bigger re-jig is needed.

There is also a more minor problem that force-clearing a conversation leaves a message in the room.