User role dropdown

Closes #3402, Closes #3428

------------------



### Components
- Added `GhostSelectComponent` to handle async select creation (h/t @rwjblue)
- Added `GhostRolesSelector` (extends GhostSelectComponent) for displaying user role options
- Created `StoreInjector` for surgically inserting the store into things that normally wouldn't have them.
- InviteNewUserModal now uses GhostRolesSelector & defaults to Author (per 3428)

### User Model
- Added `role` property as an interface to getting and setting `roles`
- Refactored anything that set `roles` to set `role`
- isAdmin, isAuthor, 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