MU Importer - Assign objects correctly

fixes #3716

- change the importer to not override any user details
- only set published_by if it is not already set
- import users before anything else
- process the import and map user ids to existing users
- test fix - owner should have owner role
- test fix - catch invalid success in importer
Merge pull request #3726 from ErisDS/issue-3716

Fixed MU Importer