Sort and remove duplicates from assignee options and save to cache

This is an ugly hack to compensate for the fact that we still don't have a proper end-point for assignable users. The end-point we are using returns a list of group_memberships, where we side-load users and need to paginate to get everything. Because users are side-loaded they are not sorted and may also be duplicated between pages.

Support ticket ID: 364307

/cc @jwswj @s-vizzari