Refactor experiment views.

Rename functionality with is abstract as mixin classes.
Reorder classes so mixins are before views.
All concrete views inherit from a single base view plus any required
mixins.
Drop use of super in get_context_data

Not really needed - just be explict about what context we want in the
view.
Drop ViewInThisApp class

It doesn't really simplify anything, so let's just drop it.
Mixin refactoring

Remove diamond inheritence.
Merge pull request #2 from tomchristie/view-refactor

View refactor