Inviting users for team admins

### Describe the problem

The invite functionality is currently only present in admin scopes (project admin / site admin), but it is not exposed to team admins to invite users to their team.

### Solution brainstorm

Include an invitation form on the team members page to allow team admins to invite new users. They currently can only add existing users. The invitations should be only allowed on sites with registration open (this matches restrictions imposed on project admins).

### Describe alternatives you have considered

_No response_

### Screenshots

_No response_

### Additional context

_No response_