Introduce a border-radius css var and use it everywhere

Sets border-radius to 4px as the app-theme default. The only
border-radii that stay different from 4px are the circle shapes for
account and voting chips.

Change-Id: I3417b141aaa19440300f0628eca1a9229eb6c131
diff --git a/polygerrit-ui/app/elements/edit/gr-editor-view/gr-editor-view.html b/polygerrit-ui/app/elements/edit/gr-editor-view/gr-editor-view.html
index 2237315..6598772 100644
--- a/polygerrit-ui/app/elements/edit/gr-editor-view/gr-editor-view.html
+++ b/polygerrit-ui/app/elements/edit/gr-editor-view/gr-editor-view.html
@@ -63,7 +63,7 @@
       }
       .textareaWrapper {
         border: 1px solid var(--border-color);
-        border-radius: 3px;
+        border-radius: var(--border-radius);
         margin: var(--spacing-l);
       }
       .textareaWrapper .editButtons {