CFD-202 : Split og vocab facet

Spliced the facet in 2 new facet blocks:
- Group Categories
- Group Tags

Added extra facetapi sorter so we can sort terms by their weight.

Added extra facetapi widget so we can group taxonomy terms by their level 0 terms (used for Group categories).

Added missing geo tags to discussions.

![c4m-discussions-overview](https://cloud.githubusercontent.com/assets/133124/5179890/1a53c16e-7485-11e4-9624-8fb57cb4ecba.png)



yeah, I understand why you do it, but that's not a pattern followed in core. I can live with this if you insist.