Implemented list page for site activity dashboards
Clean up templates in dashboard list pages

Two templates varied only by a small amount of copy, so refactor to allow them to share a single template.
Remove hard-coded "web traffic" from site activity dashboard titles

This can be added dynamically in the dashboard view and helps keep the dashboard list clean
Dynamically create department and agency lists

Instead of creating files at start-up, dynamically create department and agency lists based on the values present in a filtered subset of the dashboards list.

This means that we only display departments and agencies in the dashboard list pages which actually have related dashboards available.
Adjust css for dashboard list pages

Styles were dependent on the exact height of the left hand column content, which broke when the dropdown menus were made dynamic. Making the left hand column position absolute means we can always stick the right hand column to the correct point.
Only write query parameters if they're set on dashboard list pages

In particular "agency" is not always in the form, so shouldn't be added to the query string.
Configure template params in dashboard list pages

So lists can have their own search examples.
Fix filtered list tests
Fix width on filtered list select boxes
Merge pull request #728 from alphagov/feature/content-dashboards-list

Feature/content dashboards list