issue TryGhost/Ghost#7452, requires TryGhost/Ghost#7451
- use a `ghostOAuth` config flag to switch between the old-style per-install auth and centralized OAuth auth based on config provided by the server
- add OAuth flows for:
- setup
- sign-in
- sign-up
- re-authenticate
- add custom `oauth-ghost` authenticator to support our custom data structure
- add test helpers to stub successful/failed oauth authentication
- hide change password form if using OAuth (temporary - a way to change password via oauth provider will be added later)
refs https://github.com/TryGhost/Ghost-Admin/pull/210
- removes unused `activeTheme` property on `gh-theme-table`
- updates label generation in `gh-theme-table` to add folder names when there are duplicate package.json name+version combos