Optimize the declaration of view actions

[jeremystretch]

Proposed Changes

ActionsMixin defines two attributes on a view:

• actions - A tuple of support action names
• action_perms - A dictionary mapping of action names to a set of required permissions for each

The following actions are defined by default:

    actions = ('add', 'import', 'export', 'bulk_edit', 'bulk_delete')
    action_perms = defaultdict(set, **{
        'add': {'add'},
        'import': {'add'},
        'bulk_edit': {'change'},
        'bulk_delete': {'delete'},
    })

These can be combined into a single mapping:

    actions = {
        'add': {'add'},
        'import': {'add'},
        'export': set(),
        'bulk_edit': {'change'},
        'bulk_delete': {'delete'},
    }

Justification

This change will simplify the list of support actions for a view and reduce duplicate code.