Update permissions for who can assign/change SE and HE
The admin user should be able to set and change the SE and HE assigned to a manuscript, but it appears that anyone can (even though changing SE or HE doesn't actually work - see #212 (closed)).
Currently, anyone with access to the ms control panel can use the dropdown to select a different HE and SE. The permissions should be as follows:
- Admin should be able to assign SE and HE as currently implemented
- SE should only be able to assign HE
- HE should not be able to change assigned SEs or HEs (HE should only be able to invite reviewers)
- Reviewers shouldn't be able to set or change SE or HE