[Glitch] Fix scroll to top in single column UI

Port 2dee293c4c to glitch-soc

Signed-off-by: Thibaut Girka <thib@sitedethib.com>

app/javascript/flavours/glitch/styles/components/columns.scss

@@ -112,6 +112,8 @@
 }
 
 .column-back-button {
+  box-sizing: border-box;
+  width: 100%;
   background: lighten($ui-base-color, 4%);
   color: $highlight-text-color;
   cursor: pointer;