Convert rendering of login.html to use a nunjucks template

Convert rendering of login.html to use a nunjucks template

This new base.html template will allow us to avoid duplicating a bunch
of boilerplate as we start adding additional HTML pages.

Also, we pull the undisplay, add_message, lmno_login, and
lmno_login_loaded functions in instead of including the static
JavaScript file from /lmno.js.

This makes things much more independent here, (rather than relying on
JavaScript functions defined in a script file that is maintained in a
separate git repository: lmno.games).