Summary: Adds a migration in preparation for future work on tracking and deleting attachments. This includes a `_grist_Attachments.timeDeleted` column which isn't used yet, and changing the storage format of user columns of type `Attachments`. DocStorage now treats Attachments like RefList in general (since they use JSON), which also prompted a tiny bit of refactoring.
Test Plan: Added a migration test case showing the change in format.
Reviewers: dsagal
Reviewed By: dsagal
Differential Revision: https://phab.getgrist.com/D3352
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
Test Plan: Wrote unit and browser tests that verify new behavior.
Reviewers: paulfitz, dsagal
Reviewed By: dsagal
Subscribers: alexmojaki
Differential Revision: https://phab.getgrist.com/D2855
