Filtering and Article Sorting

Added functionality to filter and sort articles along with other bug
fixes.

- bulk importing from any post type: now on any post list in WordPress
there’s an option in the bulk action drop down to import it as an
article into the DPS plugin
- removed the ability for the plugin’s AJAX calls to be called outside
of the backend admin
- article lists inside modals can be filtered and searched
- the main article list in the plugin can be filtered and added the
ability to bulk delete or assign a parent article to an issue
- added classes for view templates (using mustache) and filtering