Extract leftover strings

Extracts strings that were missed previously in:
- Dashboard page of admin interface.
- Edit Custom CSS modal of admin interface.
- Settings modal of admin interface.
- Post activity list on user page of forum UI.
Hopefully there aren't any more!

js/admin/src/components/SettingsModal.js

@@ -33,7 +33,7 @@ export default class SettingsModal extends Modal {
         className="Button Button--primary"
         loading={this.loading}
         disabled={!this.changed()}>
-        Save Changes
+        {app.translator.trans('core.admin.settings.submit_button')}
       </Button>
     );
   }