Sort and remove duplicates from assignee options and save to cache

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 contain duplicates between pages.

JIRA: https://zendesk.atlassian.net/browse/APPS-558

/cc @jwswj @s-vizzari