add_terms() replaces add_term() and add_free_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).

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.

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.

Refactor views into multiple files prior to developing edit item features. Also changed the edit/create item urls.