In order to make the layout of all bots consistent, this commit
moves each bot into a folder with its name and modifies 'run.py'
so that only bots with such a structure can be executed. 'lib'
gets renamed to 'bots'.
This commit also starts to build out the infrastructure for
helping Zulip contributors to more easily author bots in
a way that sets up for running some bots on the server itself.