(core) Disable overscroll in gridview

Summary:
This blocks browser back/forward trackpad gestures from unintentionally
triggering when the cursor is over an overflown gridview. The gestures
are still allowed elsewhere in the UI, as well as in gridviews that have
not overflown.

Test Plan: Tested manually.

Reviewers: paulfitz

Reviewed By: paulfitz

Differential Revision: https://phab.getgrist.com/D3474

app/client/components/GridView.css

@@ -19,6 +19,7 @@
   height: 100%;
   width: 100%;
   overflow: auto;
+  overscroll-behavior: none;
 
   z-index: 2; /* scrollbar should be over the overlay background */
   border-top: 1px solid lightgrey;