Badge system

Implements (mostly) priorities #1-#5 listed here: https://meta.discourse.org/t/badges-feedback/13789/13

Pending:

1. Redirecting /badges/:id/wrong-slug to the correct URL.
2. Pagination/infinite scroll of users on the badge page instead of showing the 20 most recent grants? Or alternately just display _all_ users who were granted the badge?