Add User Role Dropdown

Closes #3402, Closes #3428

-------------------

 ### Components
- Added GhostSelectComponent to handle async select creation (h/t @rwjblue)
- Added GhostRolesSelector (extends GhostSelect) for displaying user role options
- Created StoreInjector for surgically inserting the store into things that normally wouldn't have them.

 ### Users Settings
- InviteNewUserModal now uses GhostRolesSelector & defaults to Author
- The role dropdown for user settings has permissions set per 3402

 ### User Model
- Added `role` property as an interface to getting and setting `roles`
- Refactored anything that set `roles` to set `role`
- isAdmin, isAuthor, isOwner and isEditor are all keyed off of `role` now

 ### Tests
- Added functional tests for Settings.Users
- updated settings.users and settings.users.user screens
- fix spacing on screens

 ### Server Fixtures
- Fixed owner fixture's roles
Merge pull request #3436 from novaugust/user-role-dropdown

User role dropdown