Capture Person/User merges going forward

[rjsparks]

Create a new pair of tables in the Person app, possibly called MergedPersons and MergedUsers, that mean "this pk was merged into that pk at this timestamp by this user".

Then we can modify the APIs for MeetEcho to follow merges when the get one pk on Monday, objects were merged on Tuesday, and when they try to upload attendance/bluesheets on Wednesday, the pks they provide now longer exist...

cc: @rpcross, @alexamirante

[rjsparks]

#11221 is the path we are planning to go down instead. Closing this issue as OBE