Summary:
Existing filters are now moved out of fields
and into a new metadata table for filters, and the
client is updated to retrieve/update/save filters from
the new table. This enables storing of filters for
columns that don't have fields (notably, hidden columns).
Test Plan: Browser and server tests.
Reviewers: jarek
Reviewed By: jarek
Differential Revision: https://phab.getgrist.com/D3138
