CONTENTBOX-157: Switch to Category modal add/edit

Updated category management page to use modal for add/edit, to emulate
new approach for roles/permissions.

Also added some logic into the general modal management to clear form
fields when a modal is cleared. This resolves some errors I noticed
with the new role/permissions management where if you edit, and then
create, the new item is actually an update of the previously-edited
item. Clearing the form fields prevents this from happening.