Hide the Contents tab by implementing INonStructuralFolder.
Add a Subcollections tab, hidden by default.
Allow to add a Collection within a Collection in the FTI.

No 'add item' dropdown is shown as Collection is a non structural folder,
so this should be fine as default.
You can control the display of the Subcollections tab instead.
Added upgrade step to apply Collection.xml.
Added a sub_collections view.

This is a copy of the folder_contents view from plone.app.content.
We inherit from the browser views there and explicitly use the getFolderContents method instead of queryCatalog.
Use 'Also in this section' as sub_collections title.

Same as the original atct_topic_subtopics.pt.
Add a link on the sub_collections page to add a Collection.
typo
Show Subcollections in standard view.
Fixed action url
Allow Collections as default page even though they are folderish
Show Subcollections tab when object/allowAddSubcollection is True.

Non-Collection objects will not allow it.