refactor: Bots are now implemented as classes

This rewrite puts all of a bot's logic into its own class.  This allows
us to have an environment variable that defines a bot's personality, and
to distribute the same codebase across multiple worker instances.

Selection of which bot personality will be done in an environment variable.
bot: Get working with an actual real server
donbot: Add option to ignore old posts

This adds a new parameter to donbot -- "ignoreolderthan" -- that allows
the bot to ignore donations that are older than a certain time.
procfile: Rename main worker class