Summary:
The Importer dialog is now maximized, showing additional column
matching options and information on the left, with the preview
table shown on the right. Columns can be mapped via a select menu
listing all source columns, or by clicking a formula field next to
the menu and directly editing the transform formula.
Test Plan: Browser tests.
Reviewers: jarek
Reviewed By: jarek
Differential Revision: https://phab.getgrist.com/D3096
