refs https://github.com/TryGhost/Team/issues/1130
- copied theme upload modal to new modals system and refactored for Octane syntax
- updated to use `themeManagement` service rather than passed in actions so the modal-based process can be opened from any screen
- added default `beforeClose()` for the modal to the modals service so it won't close if an upload is in-progress (defaults were moved directly into the class so it had access to services)
- added `themeManagement.upload` action for triggering the upload modal and providing a central place for limit checks
- added upload-triggering buttons to change-theme and advanced design screens
