Summary:
Adds Reference List as a widget type.
Reference List is similar to Choice List: multiple references can be added
to each cell through a similar editor, and the individual references
will always reflect their current value from the referenced table.
Test Plan: Browser tests.
Reviewers: dsagal
Reviewed By: dsagal
Subscribers: paulfitz, jarek, alexmojaki, dsagal
Differential Revision: https://phab.getgrist.com/D2959
