Update subject priority for sequential selection

[yuenmichelle1]

Part of this panoptes issue: zooniverse/panoptes#4582

TL;DR of the issue is that when project teams want to set up sequential subject selection BUT did not upload priority within subject metadata on initial upload, Designator does not see updated priority when project team updates subject metadata. [Eg script of how project teams update subject metadata found here: https://github.com/zooniverse/Data-digging/blob/master/scripts_Utility/edit_metadata_indiv.py] Mainly because Designator looks at a Subject's corresponding SetMemberSubject to view priority.

This change allows project teams a way to not only update subject metadata's priority but also the SetMemberSubject for Designator to show correct subject order.

TESTING CHECKLIST

• Priority gets updated for both metadata and corresponding set_member_subject
• As priority is updated for a workflow that uses sequential subject selection, Designator displays the correct subjects in priority order.
  --- Note for ^ , workflow might need to be reloaded on Designator for subjects to show up in correct priority order.
  (via authenticated POST to /api/workflows/:id/reload)

TODO

• Example script on DataDigging repo (PR: )