Take a refactoring pass

This was a bit of whirlwind but the vague list is:

  * Put everything into librehosters.config.Config
  * Refactor librehosters_cli.commands.* to use new Config
  * Ensure both 'missing' and 'unknown' schema key validation
  * Use only click.UsageError (not click.ClickException)
  * Store constants in librehosters_cli.settings

I think that's it then!

Signed-off-by: Luke Murphy <lukewm@riseup.net>