* Add contributor management to manage form
* Check `pledge_id` to prevent returning all contributors
* Return a plain text error string – this is used in an alert box, so it can't contain HTML
* Hide confirmation when pledge is a draft
* Only enqueue script if the user is authorized
This updates the display of contributors into a table view, and adds the ability to add and remove contributors to existing pledges.
The display has been refactored to use JS templates & JSON contributor data– the data is output onto the page when loaded from the server, and rendered when the page finishes loading. Adding & removing contributors now submits to an admin-ajax.php endpoint, which, if successful, return the new list of contributors. This ensures the display is always up to date.
Fixes#3
