Update for synchronous feature service
Add client-side handling of server-side errors when adding subscribers
- display server-provided error message when we get a server error
- fix the ajax util's `getRequestErrorMessage` method so that it works correctly with Ember's `InvalidError` object instead of the previous request object that it was receiving (*TODO:* this really needs looking at properly so we aren't losing details and Ember Data can do it's stuff)
Styling updates
- proper icon for ascending/descending
- change hover colour to green for "Import CSV" button
Delete subscriber button with confirm modal
- display delete button when hovering over a subscriber row (WARN: really ugly button, styles definitely want looking at)
- show confirm modal when clicking the delete button
- delete subscriber, remove from table, and update total on confirm
