[Glitch] Use heading actions and placeholders in settings

Port SCSS changes from 36426ed4ad to glitch-soc

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

app/javascript/flavours/glitch/styles/admin.scss

@@ -894,3 +894,7 @@ a.name-tag,
     color: $primary-text-color;
   }
 }
+
+.center-text {
+  text-align: center;
+}