Share studies issue 476

This allows studies to be shared between users (addresses part of #476).  Notes:

* Adds a "Modify" button in the sharing column for "My Studies"
* Clicking the button shows a modal with a Chosen multiselect
 * The multiselect has all users available. It is updated via AJAX (and async calls in Tornado) to have selected all users with whom the study is currently shared
 * Adding or removing people to the Chosen multiselect updates (again via AJAX and async) the users with whom the study is shared
 * The Sharing column of the My Studies table is also updated dynamically based on the latest changes the user makes to the sharing settings
* If a study is public, sharing settings cannot be modified.
* A study cannot be shared with the study's owner
use `self._id` for consistency sake.