Editor object permissions in a multisite

[MyPyDavid]

Implementation for #490 with object level permissions and an "editor" per site.
The additions:

• editor and reviewer to user Role , m2m with Site
• editors field to all element models, m2m with Site
• object permission rules for all elements, checks are based on the editors and user Role
• test fixtures for foo.com and bar.com and users e.g. foo-editor, bar-editor, ...
• the HasObjectPermission to the Viewsets
  • included tests for this
• the read_only field to the Serializers
  • via the ReadOnlyObjectPermissionsSerializerMixin class in rdmo/core/serializers.py
• read_only displayed in front-end templates.

---

Status, changelog

• change target base branch into dev-1.10.0
• change target base branch into dev-2.0.0
• merged dev-2.0.0+refactor_elements into feature/490-editor-permissions-multisite, thank you @jochenklar ;)
• fix the todos from after the merge
• add the editors selection and read_only functions to React front-end
• fix remaining tests, add fixture
• other todos..

[coveralls]

Coverage: 90.692% (+0.2%) from 90.469% when pulling 56cce83 on feature/490-editor-permissions-multisite into 813df5e on dev-1.10.0.