Refactor add_items_categories and add_categories into a single call, and make the taxonomy slug configurable at that level (it's now hard-coded in process_items). Update tests to match the change and ensure we can update multiple items at the same time.

Make the default empty batch command a line of dashes under the 'Actions' header. We can't really do better than that without js.

Re-introduce the per-row drop down to update each record's category as well as the mirrored buttons at the bottom of the page. This means that we have a single form that can either do a batch update, or an update of values set per row. The different action is determined by the submit button that was used - which is always named 'action'. Because we have a mirrored select drop down for the batch action, we need to know which one to take into account - so the value of the submit button indiciates which one was clicked, of the form <action>-<placement> (eg. save-top) and the name of mirrored inputs indidicate where they are located, of the form <name>-<placement> (eg. batchaction-bottom). The ViewItems class has a static method to find the active placement and rename the parameters accordingly. Some of the old code was re-introduced, though some of it was refactored (eg. what used to be get_categories, to fetch each row's selected category, has been moved to ItemTable to parallel get_selected, and is now named get_row_select_value (it's a more generic version, looking at the future when we'll update the region with the same mechanism.)

Now raises TransportException if taxonomy does not exist

Now checks if taxonomy exists

In order to support the next test, which will use a slug of a
taxonomy that doesn't exist

remove unused import and fix pep8 white space

Rename select_action to select_item which makes more sense. Rename the checkbox itself select_item_id to allow other types of selectes in the future. Move the code to determing selected items into ItemTable since this is where the form is generated, and we know how it was build.

Names are now tested elsewhere

The Data Layer apps need to prceed the local ones because of
dependencies, the app can depend on the data layer and imports
them before they have been initialised

https://trello.com/c/Q9hEiE20/83-change-categories-available-to-match-those-specified-in-the-story