Work on a generic policy component.

[terjekv]

Allow policies generically across models in mreg.

Policies allow for roles and atoms to be associated with objects in a given model. This PR aims to allow of such components to be defined by the users of mreg and not be hard-coded into the system on a per-model basis. The constraints are:

• Policies will only be applicable to concrete (non-abstract) model in mreg.
• Roles and atoms may be created against models that have been approved for policies. Doing this requires adding an entry into an approval model (ApprovedModelForPolicy) via POSTing {"content_type": "modelname"} to the endpoint /policy/approvals/.
• Model names for ApprovedModelForPolicy are case insensitive (they are stored in lowercase by Django), and the application label (mreg. is implied) so one does not use mreg.Host.
• A list of supported models should be made available in the documentation, especially as they are deeply tied to the current database schema.

PR status:

ApprovedModelForPolicy:

• Implement the model.
• Set up views/serializers/etc for CRUD operations
• Set up endpoints
• Test endpoints, with a specific eye towards permissions (only super + policy admins are allowed write, everyone else read)

Roles:

• Implement a generic role model
• Set up views/serializers/etc for CRUD operations into the role model.
• Set up endpoints
• Test the role model
• Test permissions of roles. Note that permissions on models that have hosts may want to utilize NetGroupRegexPermission.

Atoms:

• Implement a generic atom model
• Set up views/serializers/etc for CRUD operations into the atom model.
• Set up endpoints
• Test the atom model
• Test permissions of atoms. Note that permissions on models that have hosts may want to utilize NetGroupRegexPermission.

Assignments:

• Implement a generic model for assigning policies to objects.
• Set up views/serializers/etc for CRUD operation for assigning policies
• Set up endpoints
• Test the policies model
• Test permissions of policies. Note that permissions on models that have hosts may want to utilize NetGroupRegexPermission.

[coveralls]

coverage: 98.48% (-0.5%) from 99.013%
when pulling 6c4b342 on generic_policy_support
into cee5c16 on master.