API: Members added to an organization are automatically assigned as administrators

[linzjax]

Steps to reproduce the error

Using httpie:

http POST http://localhost:8000/api/v1/organizations/<organization>/users/ 'username'=<username> AUTHORIZATION:'Token <token>'

Check newly added users admin status

Actual behavior

Users are assigned admin status automatically.

Expected behavior

Users are assigned as members, and must explicitly be made admins.

[ian-ross]

This and #603 sort of go together, so I'll pick this one up too.