Add user permission to BadgeKit when a badge is awarded

This allows hookshot to grant permissions in BadgeKit to users who are awarded a particular badge (currently defaults to the slug "webmaker-super-mentor").  Currently this only supports a single (configurable) badge, and always awards application review privileges exclusively.  Future improvements might include the ability to configure the permissions granted for an arbitrary number of badges.

This adds several new environment variables:

`BADGEKIT_API_SYSTEM`: the system within BadgekitApi that this hookshot instance is associated with (should generally be 'webmaker')
`BADGEKIT_USER_API_URL`: the full URL of the BadgeKit (not BadgeKit API) instance's User API endpoint.  e.g. `https://badgekit.mofostaging.net/api/user`
`BADGEKIT_USER_API_SECRET`: the BadgeKit User API secret.
`MENTOR_BADGE_SLUG`: the slug of the badge that should award review permissions in BadgeKit.  Defaults to 'webmaker-super-mentor'