Fix authorizations widget issues

* Main issue: update checkbox state after replacing `this.join_list`,
  since this `replace()` operation causes the options to be redrawn
* Also remove `update_list_target`, since we never use `tmpl`-style list
  templates anymore
* Also ensure checkbox state only changes after server confirmation
* Also don't use `getRootModelName`, just `shortName`
Merge pull request #369 from reciprocity/feature/authorization-checkboxes

Fix authorizations modal issues