Show traffic priority on mappings view

Add a new **Sort by** option to the mappings index, and when sorting by hits show a **Hits** column with a descending hit count and percentage of total hits on the site, over all time.

We've acknowledged that:

- the filtering and sorting code is not ideal and could do with some [refactoring](https://www.pivotaltracker.com/n/projects/887836/stories/70586196). An example of how this might look is in 4f90fbb9fb38156b55c90c8fc14e31ae800ccfe3.

- the non-js code doesn't (and didn't):
    - keep your parameters when you go back to the filter form via the **Filter mappings** link; we thought that fixing this existing issue was outside the scope of this story.
    - say what you're sorted by or if you're filtering by type; we tried to do this but found that it would require much of the refactoring to be done before it would make sense. We also don't have the visual vocabulary to do this, so need to talk to @fofr but don't think it should block this.