Add ability to change/set slug for badges

[k88hudson]

Current badgekit-api forces a UUID for the badge slug:

https://github.com/mozilla/badgekit-api/blob/master/app/routes/badges.js#L325

[k88hudson]

Note that this has the potential to create duplicate badges with the same slug, so a check should be performed (or at least, the field should be unique as per the database schema)

[ScottDowne]

#1322