Fix norm/proposal assignment with relative_urls

Addendum to e74eaf6b0f0bda02dd3a69ac2e0065c6071c6503
Show category in proposal information box
indent
Allow assignment of categories to pages

It is now possible to assign categories to pages. The categories to be
assigned are the same categories as the proposal categories, as these
CategoryBadges are DelegateableBadges anyway, and can thus be used for
pages as well.

This way these categories may act as another bridge between proposals
and pages. If this isn't always desired, additional attributes can be
added later to the CategoryBadge model in order to define, whether a
category is meant as a proposal category, a badge category or both.

This commit doesn't add category facets to the page listing.