Collections' Work admin requires adminsortable2 app

Include the `adminsortable2` third-party app in
those installed for Collections because it is used
by various WorkCreator inlines, which will raise
500 `ValidationError` errors on form submit if
this app is missing and its required templates
therefore cannot be found.

NOTE: Given the comment below found in the
`work_creator.admin` module, we should probably
ditch this third-party requirement altogether:

    # Some super-weirdness means that this inline doesn't appear on
    # docker-cloud staging if SortableInlineAdminMixin is enabled.
    # Giving up for now. TODO: reinstate, or choose a different sorting ui lib
    # SortableInlineAdminMixin,

icekit/project/settings/_base.py

@@ -875,6 +875,7 @@
     # collections. Other types of museum may need to disable this.
     'glamkit_collections.contrib.work_creator',
     'glamkit_collections.contrib.work_creator.plugins.links',
+    'adminsortable2',  # Required by work_creator.admin
 
     # Sponsors
     'glamkit_sponsors',