(core) Forms improvements: mouse selection in firefox, focus, and styling

Summary:
- Remove unused Form file (Label.ts)
- Fix Firefox-specific bug in Forms, where mouse selection wasn't working in textarea.
- Focus and set cursor in textarea on click.
- Save on blur but only when focus stays within the Grist app, as for editing cells.
- Make paragraph margins of rendered form match those in the form editor.

Test Plan: Tested manually on Firefox and Chrome; relying on existing tests that nothing broke.

Reviewers: jarek

Reviewed By: jarek

Differential Revision: https://phab.getgrist.com/D4281

app/client/ui/FormPage.ts

@@ -165,7 +165,7 @@ const cssFormContent = styled('form', `
     font-size: 10px;
   }
   & p {
-    margin: 0px;
+    margin: 0 0 10px 0;
   }
   & strong {
     font-weight: 600;