Add sort key to default mobile api fields

Because the mobile clients will merge together plot and tree stewardship,
we need to signal to the clients how to sort the merged lists of
stewardship events.

We have no mechanism for changing collection UDF field
definitions at the moment, but if/when we add it this will cause us to need
to take care to not allow the definitions of plot and tree stewardship to
get out of sync
Merge pull request #1504 from maurizi/fix-duplicate-instanceusers

Add sort key to default mobile api fields