External user groups auth source dropdown menu doesn't open
When editing a user group, the auth source dropdown menu when adding an external user group doesn't open.
1. Edit a user group
2. Go to the "External groups" tab
3. Click "Add external user group"
4. Try to click the "Auth source" dropdown menu
Similar to #14041 I think, where select2 is being initialised in the form template when it isn't visible and before it's copied to the new form, and it should only be initialised after the new set of form fields are added.
fixes #14200 - exclude form templates from select2 initialisation
Form templates shouldn't be initialised with select2, as the elements
are copied through add_child_node, e.g. when adding a new external user
group inside the user group form. add_child_node calls select2 after
copying the form instead.