add_terms() replaces add_term() and add_free_terms()

add_terms() returns 400 if term does not exist and vocabulary is not
open

If the taxonomy has an open vocabulary, we create missing terms
rather than throw an exception

Replaced 'add_term' and 'add_free_terms' with 'add_terms'

Two error conditions:
Multiple terms passed from different taxonomies
Multiple terms passed when taxonomy does not support multiple terms

This matches the field name and simplifies the code somewhat

Unfortunately bootstrap-tagsinput does not have a configurable
delimiter so settle with ',' for now (no commas allowed in tags).

This allows the JavaScript to display the tags to work

I had originally thought the field could be called terms[free-tags] and
we could use this notation for any other taxonomy-related field. This
would be too difficult in Django so I've used the name tags as was
already in the template.

When the form is submitted, any existing free tags will be deleted
before the ones present in the form are added.

Ability to add multiple terms from the same taxonomy to an item.

It is now possible to add multiple terms to an item in the same
taxonomy via the REST API. Terms not already present in the database
will be created.

We got bad performance degrading as the number of terms increased.
Enabling django_debug_toolbar it was easy to see a bunch of queries
getting terms related to items duplicated as many times as items
in the system. Making the api view for listing items prefetch related
terms (and their taxonomies) removes these O(n) queries

The next hidden parameter in the form, which determines the redirect
following an update is set to the 'next' query parameter in the form.
If the query parameter does not exist, the HTTP_REFERER is used.
If neither exist, the user gets redirected to the dashboard.

Add the add item functionality. Also enforce a default value for item-type on forms. While the API allows for items without an item-type, the form's handling of it was inconsistant. Until we have a use case for items without an item type, a default is enforced.