Fix file validation for importer

refs #3339
- wrong call to file validation
- refs the issue because I get application/json on OSX? Is this
something Windows specific?
Integration tests use consistent teardown methods
Caching db config in migration utils
Fix tests tripping spam protection
Add relationships to client models

No issue
- Removed tabs from tag.js (why didn't jshint catch this?)
- Removed superfluous `activate` in SettingsIndexRoute
- updated `UserModel` and `TagModel` to have `created_by, updated_by` be references to `user` objects.
- updated `UserModel` to use `moment-date` instead of `date`
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 #3340 from sebgie/issue#3339

Fix file validation for importer
Merge pull request #3345 from novaugust/model-relationships

Add relationships to client models
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
Add edit roles

refs #3087
- added ability to edit user/roles relation
- user is not allowed assign roles to himself
- only one role per user is supported atm
- added tests