Refactor dashboard views to inherit from base view

Removes all the if statements to render dashboard elements based on dashboard type. Instead instantiate different dashboard views depending on the type.
Use slug for checking for top-level site activity dashboard

Instead of using the page title, because the slug is far less likely to change.
Stop using "other" nested property for tagline

Start using only the top level tagline property.
Make corresponding change to dashboard configs.
Merge pull request #498 from alphagov/bugfix/content-dashboard-breadcrumbs

Refactor dashboard views to inherit from base view