Keep the chosen folder showing in the form's Folder dropdown

[doomlab]

Closes #537

Problem

On a form's edit page, picking a folder from the Folder dropdown saved it, but the dropdown snapped back to the old folder, so it looked as if nothing had happened (confusing, as the reporter notes, with many folders).

Cause

The dropdown's value came only from the currentFolderId prop, which is the folder the form had when the page loaded. FormPlayground doesn't pass an onUpdate back to it, so after a change the prop never changed and React reset the select to the old value, even though the change had been saved.

Fix

FormFolderSelector keeps its own selected folder: set immediately when you choose (including "No folder" and a newly created folder), still follows the prop if the page does pass a new one, and put back to the previous folder with an error message if the save fails (rather than showing a folder that wasn't saved).

Testing

FormFolderSelector.test.tsx (6 tests): shows the form's folder; keeps the chosen folder when the parent never passes it back; No folder; reverts with a message if saving fails; follows a changed prop; a new folder gets selected. 4 of them fail against the old component. Typecheck (apart from the existing getJsonSchema.test.ts errors), lint and the full suite pass. Not yet checked in a browser.

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