Factor out checking of authentication

Task #333
Add servlet to check authentication


Task #333
Redirect the user to a non-cached page when authentication is needed.

This is checked both on the top page and upload.

Fixes #333: The subjective web application doesn't handle password
changes well
http://github.com/jpschewe/fll-sw/issues/issue/333
Merge branch 'task.333.subjective-password-change'