expand category

*Note that this is rebased against #666; To get correct CSS you also have to merge #687 and #694*

This adds another way to navigate proposals and norms by adding views for categories. Categories now act (much like milestones) as containers for proposals.

The design required to have an image and an extensive description for each category. This of course means that categories and thumbnail badges get even harder to distinguish. I am also not sure if the way I enabled extensive descriptions (switching from `Unicode(255)` to `UnicodeText` in the badge table) is the best way to do it.

Another thing I was not sure about is how to flatten the hierarchical category structure. I went with only displaying the leaves because, from what I saw, you can only select leave categories on proposal edit.