fix(vertical-filmstrip): scope previous Firefox scroll fixes (#2276)

In aa93a783, remote videos not scrolling on Firefox, IE, and
Edge was worked around using CSS. The CSS selector is too vague
and also catches horizontal filmstrip. The selector should
apply to vertical filmstrip only.

css/_vertical_filmstrip_overrides.scss

  * being to remove the hacks as the spec is supported.
  */
 @mixin undoColumnReverseVideos() {
-    #remoteVideos #filmstripRemoteVideos #filmstripRemoteVideosContainer {
-        flex-direction:column;
+    .vertical-filmstrip {
+        #remoteVideos #filmstripRemoteVideos #filmstripRemoteVideosContainer {
+            flex-direction: column;
+        }
     }
 }