Fixes https://github.com/flarum/core/issues/3169#issuecomment-979470794
- Restore wrapping `scope.render` results in a table cell tag. This was accidentially introduced in 5a26dd8c4b, and caused the issue linked above
- Rename the `SettingDropdown` attr `key` to `setting` in order to avoid naming clashes with Mithril vnode keys. `key` still works, but is deprecated.