Upgrade to use select2 version 4.

The MR refactors the library to use select2 version 4.0.6-rc.1 for it's user/group selection control. This is a breaking change as this version of select2 requires that the control's tag be a <select> instead of an <input>. The group_list/user_list parameters have been deprecated in favour of using the <option> tag. Also validate_crsids() has been deprecated in favour of validate_crsid_list() and validate_groupid_list() has been added.

closes: #11 (closed)