Made a bunch of improvements to admin actions. Be warned: this includes one minor but BACKWARDS-INCOMPATIBLE change.

These changes are:

    * BACKWARDS-INCOMPATIBLE CHANGE: action functions and action methods now share the same signature: `(modeladmin, request, queryset)`. Actions defined as methods stay the same, but if you've defined an action as a standalone function you'll now need to add that first `modeladmin` argument.
    * The delete selected action is now a standalone function registered site-wide; this makes disabling it easy.
    * Fixed #10596: there are now official, documented `AdminSite` APIs for dealing with actions, including a method to disable global actions. You can still re-enable globally-disabled actions on a case-by-case basis.
    * Fixed #10595: you can now disable actions for a particular `ModelAdmin` by setting `actions` to `None`.
    * Fixed #10734: actions are now sorted (by name).
    * Fixed #10618: the action is now taken from the form whose "submit" button you clicked, not arbitrarily the last form on the page.
    * All of the above is documented and tested.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@10408 bcc190cf-cafb-0310-a4f2-bffc1f526a37