Additional mock api functionality

**WIP: Do not merge**

- [x] `api.invitation.send`
- [ ] `api.invitation.get`
- [ ] `api.user.createChildAccount`
- [ ] `api.user.promoteChildAccount`

This will be merged into `master` (only mock, and new functionality, so no breaking changes). Master will then be merged in `access-and-invitations`, so this functionality can be used there.