- Add preferred provider for authorization to login and signup pages.
To use, the 3rd party application would have to add `preferred_provider=...`
parameter to OAuth2 authorization request.
- Resize 3rd party provider icons
- Add "login to authorize" heading to login and signup screens
Login may redirect to ouath2_authorizations#create which may then
redirect to arbitrary schemes if the application is already authorized
so we need to allow login to redirect to any scheme.
Fixes#3424
