This allows users to add their own local customizations to their
development environment

Closes ~sumner/standupbot#25

Add a `space.nevarro.standupbot.on_behalf_of` field to all posts to the
standup room. This can be used by other bots to know which sender the
bot is sending the message on behalf of.

This makes the bot less statefull, and takes advantage of the fact that
all of the settings are already stored as state events in the user's DM
with the bot.

Also fix setting of the config room