Summary:
Bundles some new document options into a JSON column.
The icon option is treated somewhat gingerly. It is intended, at
least initially, to store an image thumbnail for a document as a
url to hand-prepared assets (for examples and templates), so it is
locked down to a particular url prefix to avoid opening the door to
mischief.
Test Plan: added test
Reviewers: georgegevoian
Reviewed By: georgegevoian
Differential Revision: https://phab.getgrist.com/D2916
