Admin Invite User to Platform

### Epic

_No response_

### Description

**As a:** admin

**I want to:** invite a user to the platform without adding them to a team even when `Allow users to invite external users to teams` is disabled

**So that:** I can control who signs up.

### Acceptance Criteria

- [ ] Admin adds email address of user via `admin/users/general`
- [ ] User receives invite email with signup link
- [ ] User can create an account and then create a team