Create API endpoint for fetching sync statuses of individual users and classes

[marcellamaki]

This issue is the compliment to the front end issues #8178 and #8179, which create the front end displays for the learner's sync status (displayed in the AppBar dropdown menu) and the new coach/admin view of the sync status overview for a class.

Data will need to be pulled from multiple places - both to determine if there is an active or failed sync, or to determine if the user is enqueued. Richard has suggested creating a new intermediary table that can be queried that contains the necessary data.

Returned data for each user should include

• user id
• queue information
• transfer session information
• time of last sync

Outside of the scope of this issue is adding any enhancements/features that will facilitate transfer session reuse or other optimizations