Fix import path of PermissionFieldFilteringMixin
DRF filters for Customer/Project role based filtering
Use Role based DRF filters for viewsets

This constrains all the HTTP GETs performed to the API.
The implementation doesn't use object level permissions anymore.
Generalize role based queryset filtering
Remove django-guardian remnants
Remove unnecessary filtering from PurchaseSerializer
Made Customer editable in REST

-  NC-87
Modifications to user model rendering.

- NC-87
Refactored current user filtering request
Increase size of the alternative field

- NC-87
Implemented filter for the managable user lookup

- NC-87
- Lookup all users that have any role in projects where user has a
  manager role
Updated user rendering fields

- NC-87
- Expose connected projects and role type
Merge branch 'feature/nc-87-part2' into 'develop'

Feature/nc 87 part2
Fixed tests for the customer modification
Updated release notes
Install django-permission
Add FilteredCollaboratorsPermissionLogic

This piece of permission logic is based on CollaboratorsPermissionLogic
concepts.
It fixes its query bug and allows to further filter the collaborator results.
Add object level permissions to Instance.
Merge branch 'feature/permissions-revamp' into develop

Conflicts:
	nodeconductor/structure/views.py
Drop explicit ProjectGroupMembership model

NC-84