Automatically load fixtures if they're not loaded

This raises an unchecked runtime exception if the content cannot be
loaded which will cause 500s. In the previous model null pointer
exceptions were raised if the fixtured had failed to load.
Remove /fixtures route

This also removes the previous fixture loading method.
Move Fixtures out of Controllers

Fixtures is no longer a controller. It's a helper class for loading
fixtures.
Merge pull request #104 from alphagov/autoload-fixtures

Autoload fixtures