ExternalLoginForm: accept `?server` param to redirect the login form to the specified instance
Summary
Fixes #1313 (closed)
Allows creating URLs like this:
- https://fe.soapbox.pub/login/external?server=gleasonator.com
- https://fe.soapbox.pub/login/external?server=http://localhost:3000
- https://soapbox.mysite.com/login/external?server=mysite.com
The external login screen will automatically redirect you to the OAuth form for the server you specify.
If the server is invalid or couldn't be reached, it'll display an error and fall back to the usual form.