Summary:
Reverse and Aggregation lookup.
Aggregation lookup works when table have a reference list column. It allow to list value of any fields of a referenced values, or to make some basic operation on them (sum, average, count)
Reverse lookup works as reverse one, but it allow do to the same operations on all rows that have reference to given row
Test Plan: Manual so far.
Reviewers: jarek
Reviewed By: jarek
Differential Revision: https://phab.getgrist.com/D4083