Ensure plugin views are subclasses of an Adjutant defined view class

Right now there isn't anything stopping a plugin from using any type of valid django view class.

We should make an BaseAdjutantView class, and ensure all plugin views are a subclass of that. While we can't stop people from doing silly things, at least we can ensure they are using a subclass of one which we know has a bunch of reasonably sensible features (which of course they can override).

Depending on how far we go with this, it would be good to stick in some validation to ensure any plugin view classes are actually not doing something too silly, and overriding functions we consider to be things they shouldn't touch.