Idea Challenges

Also known as banner - add the ability to associate an idea with a challenge campaign.

* Add title field to banner model, title=50 chars, text = 2000 chars
* The create-idea page has a drop-down to select the associated challenge (optional)
* The main page has a "banner" for the current challenge, with a link to the challenge detail page
* The challenge detail page lists more information about the challenge, as well as all ideas and tags associated with the challenge.
  * This is not finalized, design mockup still in progress
* Other style cleanup
  * Remove total idea count from front page
  * Use common header style for non-front-page pages to match style of other collab apps
  * Tab bar black underline changed from 5px to 1px thickness