(core) Revamp attachment editor + preview UI, and support more types, including PDFs.

Summary:
- New UI for the modal look mostly following the design prepared previously.
- Use <object> for rendering PDFs (on a Mac works on Firefox, Chrome, Safari; needs checking on Windows)
- While we are at it, use <video> and <audio> for relevant files (object would
  render them too, but without the option to disable autoplay).
- Use <object> for unknown types, except for text/html (unsafe) and other text
  types (need more work to render well).
- Fix skipping save on Escape or when attachments are unsaved (previously a
  noop action was emitted, creating surprises with undo).
- Display extension for files without preview, both in-cell and in the modal.
- Replace tiny "eye" icon to preview particular attachment with double-clicking.
- As an accidental feature, a particular attachment can be previewed by typing 1, 2, 3, etc into cell.
- Renamed PreviewsWidget/PreviewModel to AttachmentsWidget/AttachmentsEditor.

Test Plan: Unified old and new tests for attachments, added new test cases.

Reviewers: paulfitz

Reviewed By: paulfitz

Differential Revision: https://phab.getgrist.com/D2667

app/client/ui2018/modals.ts

@@ -47,9 +47,11 @@ export type ModalWidth =
 export function modal(createFn: (ctl: IModalControl, owner: MultiHolder) => DomElementArg,
                       options: IModalOptions = {}): void {
   function close() {
-    document.body.removeChild(modalDom);
-    // Ensure we run the disposers for the DOM contained in the modal.
-    dom.domDispose(modalDom);
+    if (modalDom.isConnected) {
+      document.body.removeChild(modalDom);
+      // Ensure we run the disposers for the DOM contained in the modal.
+      dom.domDispose(modalDom);
+    }
   }
 
   const modalDom = cssModalBacker(