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
Extend adapter to support automatic includes

Closes #3325
- Add Roles model and add hasMany roles to User model.
- Add EmbeddedRelationAdapter that will automatically include
  hasMany relations in calls to the API.
- UserAdapter and PostAdapter now extend EmbeddedRelationAdapter
  and all explicit includes from store.find() have been removed.
Pagination for Users Management screen

closes #3222
- implementing server-side pagination for /users API
- passing /users?limit=none will return all users
- passing /users?status=invited will filter base on user status
- creating 3 mixins (route, controller and view) to keep pagination logic DRY
- updating route, controller and view for Posts to use new mixing
- implementing infinite scrolling for Users Management screen (using new mixins)
- Users Management screen displays all invited users, but paginates active users
Fix check for using default cover image

Closes #3348
- Cover attribute is a string so its typeof will always return
  string. Switch check to Ember.isBlank.
Merge pull request #3349 from jaswilli/issue-3348

Fix check for using default cover image
Merge pull request #3311 from javorszky/iss3196

Added /roles/ API endpoint
Merge pull request #3337 from morficus/issue-3222

Pagination on the Users Management screen
Merge pull request #3346 from jaswilli/issue-3325

Extend adapter to support automatic includes
Only load posts once on navigating to content tab

No Issue
- Loading posts from the API should not be necessary in PostsIndexRoute
  because its parent resource (PostsRoute) pre-loads the store.
  Changing the store.find to store.all gets rid of a duplicate
  network request to load all posts.