Generate the login/logout link server-side instead of with javascript

Generate the login/logout link server-side instead of with javascript.
    
I added a template variable `loggedinuser :: String` which is set to the name of the logged in user if there is a user logged in, and is unset otherwise.
    
This means the login/logout links in the userbox can be generated server-side now, and the javascript previously used is redundant. I changed the userbox code and removed the j
    
This techically changes the public API, as `filledPageTemplate` now takes a `Maybe User` argument which it didn't take previously. Probably the version in gitit.cabal should be