Extend sshkey filtering capabilities

- ITACLOUD-2449
Extend User model sorting

- support descending sorting as well
Make permission serializer user output consistent
Minor fixes to User serializer/filter
Extend ssh key save validation

- ITACLOUD-2447
- Emit meaningful message on unique constraint violation
- Basic support for public key format validation
Extend project filtering/sorting with group names

- ITACLOUD-2452
Merge branch 'feature/filtering-fixes' of https://github.com/opennode/nodeconductor into feature/filtering-fixes
Merge branch 'develop' of code.opennodecloud.com:nodeconductor/nodeconductor into features/filtering-fixes

Conflicts:
	nodeconductor/structure/serializers.py
ssh key tests and serializer validate method
Merge branch 'feature/filtering-fixes' of code.opennodecloud.com:nodeconductor/nodeconductor into feature/filtering-fixes
Merge branch 'feature/filtering-fixes' into 'develop'

Feature/filtering fixes