Fix sidebar tag filtering, disable new ideas for inactive challenges

* ability to select/unselect multiple tags
 - selected tags are highlighted using ".pushed" class promoted by collab
* idea list correctly filters for the selected tags
* tag count shows count for the current filter
* challenge page only shows tag count for the ideas in the current challenge, not global counts
* challenge page only shows `new idea for this challenge` link for active challenges
* add tests to ensure only active challenges are available in the add idea page