Summary:
This adds a UI panel for managing webhooks. Work started by Cyprien Pindat. You can find the UI on a document's settings page. Main changes relative to Cyprien's demo:
* Changed behavior of virtual table to be more consistent with the rest of Grist, by factoring out part of the implementation of on-demand tables.
* Cell values that would create an error can now be denied and reverted (as for the rest of Grist).
* Changes made by other users are integrated in a sane way.
* Basic undo/redo support is added using the regular undo/redo stack.
* The table list in the drop-down is now updated if schema changes.
* Added a notification from back-end when webhook status is updated so constant polling isn't needed to support multi-user operation.
* Factored out webhook specific logic from general virtual table support.
* Made a bunch of fixes to various broken behavior.
* Added tests.
The code remains somewhat unpolished, and behavior in the presence of errors is imperfect in general but may be adequate for this case.
I assume that we'll soon be lifting the restriction on the set of domains that are supported for webhooks - otherwise we'd want to provide some friendly way to discover that list of supported domains rather than just throwing an error.
I don't actually know a lot about how the front-end works - it looks like tables/columns/fields/sections can be safely added if they have string ids that won't collide with bone fide numeric ids from the back end. Sneaky.
Contains a migration, so needs an extra reviewer for that.
Test Plan: added tests
Reviewers: jarek, dsagal
Reviewed By: jarek, dsagal
Differential Revision: https://phab.getgrist.com/D3856
