Changes to the UI

- login screen looks a bit fancier and is more customisable
- admins can add a Logo next to the login fields, there's a MISP logo
ontop with a line of text above and below it, editable via bootstrap.php
- Footer re-added, has the PGP key download and the center footer text
from MISP 1.1
- A logo on the right side of the footer, optionally added by
bootstrap.php

- Header, Footer, menu are now fixed and not affected by scrolling the
screen