R1:2085716da6d1

Make dashboard arrange actions (move, add, remove) work again after read locking from "chaos reduction"

Summary:
See PHI385. Ref T13054. Ref T13083. The dashboard "arrange" operations (add, remove, move) rely on doing `$dashboard->setThing(...)` and then applying transactions.

This no longer works after the read locking change from T13054. To make this function again, just add an explicit `save()` after layout adjustment. This should be more nuanced eventually, but all arrange operations are…