Summary:
Changes include:
* Hide the colum matching section for new destinations (for now).
* Make the preview table read-only.
* Don't show helper column IDs when the formula editor is open.
* Fix the formula editor autocomplete to show suggestions
from the active transform section.
* Hide the formula icons in the preview table, and other unnecessary
UI elements such as row dropdown menus.
* Keep preview loading spinner shown if scheduled (i.e. debounced) diff updates exist.
Test Plan: Browser tests.
Reviewers: paulfitz
Reviewed By: paulfitz
Differential Revision: https://phab.getgrist.com/D3148