Improve consistency of how views in ModelAdmin are initialised

[ababic]

In modeladmin, there's a bit of inconsistency in how the views are initialised. There's a distinction between 'Instance specific' views (those that sub-class wagtail.contrib.modeladmin.views.InstanceSpecificView) and the other views.

Really, there's no need for the __init__() method of InstanceSpecificView to be any different to that of WMABaseView (the only important thing for modeladmin views at initialisation is the linking to a ModelAdmin instance). The instance_pk should be provided to dispatch() instead, and dealt with appropriately there. This would give us:

• A way to truly 'genericise' the various action_view() methods on the ModelAdmin class (as currently being proposed in Simplify adding new views to ModelAdmin #10) without having to add special treatment for certain actions.
• Views that are easier to understand, and easier for developers to reuse.
• Fetching the object in dispatch would mean we have access toHttpRequest object, so we could reuse ModelAdmin.get_queryset() to get a base queryset from which to search for an instance, rather than just using the default manager's get_queryset() method.

The only thing I'm not sure about is backwards compatibility / deprecation, since we're currently telling people in the docs to copy from current examples to implement custom views... so that's potentially a big issue.

Any input / thoughts from others would be much appreciated.