Added /roles/ API endpoint

Closes #3196

* adds `/roles/` endpoint
* is given the current user as context
* wraps everything in a canthis.browse.role
* gets all the available roles (should "Owner" be filtered out?)
* optional parameter: `permission=assign`. Gets all roles authenticated user could assign
* if we're not signed in, gives a "please sign in" (standard) error
* if we're signed in, but user is not in the context, gives a "there was no user in the context" error
* if the user is an "Author", gives a "there are no available roles to assign" error
* implemented hacky filter because when.js produces heisenbugs past 3.2.3 (when.filter not available)
* added extra fixtures to `permissions.json`. Might need a migration.

Caveats:

* there are no tests
* for some reason the setup functional test was failing for me locally
Merge pull request #3311 from javorszky/iss3196

Added /roles/ API endpoint