Also:
- Add CSS to login page
- Refactor login page HTML and CSS into a new package
- New custom CSP headers for the login page, because the requirements
are different from the form_post page
The other handlers for GET and POST requests are not yet implemented in
this commit. The shared handler code in login_handler.go takes care of
things checking the method, checking the CSRF cookie, decoding the state
param, and adding security headers on behalf of both the GET and POST
handlers.
Some code has been extracted from callback_handler.go to be shared.
