refs https://github.com/TryGhost/Team/issues/1130
- added new route for viewing themes so back/forward buttons can be used
- takes theme name as a parameter
- opens a fullscreen modal with an iframe containing the theme demo
- changed installable themes list to link to new route
- swapped `previewUrl` in theme data to point at the real demo rather than the ghost.org demo page
refs https://github.com/TryGhost/Team/issues/1130
- match latest design where the theme table is under an "advanced" toggle on the change-theme screen rather than a completely separate screen
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
refs https://github.com/TryGhost/Team/issues/1130
- added link to design sidebar
- dropped change-theme modal and associated route behaviour
- copied basic themes marketplace list into the change-theme controller/template
