Invalid invite fields when creating invite as a moderator

[mcwumbly (Dave McClure)]

@Zeemah

Despite this not allowing the moderator to do so (returning a 403), the drop down box still is available

The drop-down should probably only show for any admin or group owner.

@Discourse

Group owners and site admins can invite new users to a group from the group’s page

(Non admin group owners should probably only be able to select a group that they own).