Only display the Remove Question Type menu item on pages where the
question type can be set.

Ideally I'd like None to represent no category but empty string fits
better with what is on the form for the individual dropdowns. So
let's make that consistent.

If there is no source set on the tab configuration, the upload
button will no longer be displayed.

So user is redirected to the correct page after uploading data

When process_items is called without a request, redirect to data-view (don't try to double guess the tab)

Provide the tab instance to get_context_data, and use that to work out the URL to redirect forms to.

Refactor the ViewsItems view, which covered the view and edit table, into a separate (non-view) class under tabs/view_and_edit_table.py. When ready, this will plug straight away into the new tabbed page functionality - in the mean time there is a place holder view to display the view where it used to be found.