Functionality for changing banner on front page through abakus.no

[ivarnakken]

As of now, the banner must be changed manually through code changes, which is neither clean nor practical. Since the banner has been receiving positive reviews and seem to have come to stay, we should introduce functionality for changing it (color, content and visibility) through abakus.no directly.

This would of course require admin permissions, however, I'm not sure "where" this should be located. Perhaps under the admin page "Kunngjøringer" or simply under an entire new admin page accessible through the hamburger menu. On that thought, perhaps it would even be nice with a new admin page solely for the front page, which could also contain functionality for pinning articles (especially needed once the new front page introduces two articles) and choosing which polls to show etc.

In addition, it's worth noting that the banner must be manually updated two places, both on the "public" and "private" front page. This is simply unpractical, and even made me forget to update one in lego-webapp/#2864.