Use badge to show number of categories in heading

The chevron didn't illustrate that the heading had any category
children. The app uses the badge pattern elsewhere.

![screen shot 2014-11-18 at 14 17 21](https://cloud.githubusercontent.com/assets/282788/5088732/e6d1eebc-6f2d-11e4-8435-93644c701693.png)

Adds minor style improvements:

  - Pad erb tags
  - Use each instead of for

Fixes https://github.com/mysociety/alaveteli/issues/1962