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.