[controller/feature] Add DeviceGroup model

[nemesifier]

In the last 2 years several features have been requested and built (I and others have built a few of these ideas with custom openwisp modules) which could be implemented/reimplemented with the concept of grouping devices.

A group could automatically assign special templates, it could automatically assign special monitoring settings, it could provide a default location, it could allow to implement the synchronized update strategy more effectively (devices of the same group can reload special type of configurations in a synchronized fashion), a group of devices could be a mesh network so the group could automatically generate all that is needed to make the mesh network work and collect network topology information. The actual things we can do with this concept vary depending on the use case.

Another use case is that the old OpenWISP1 version had the concept of "group" for specifying group of access points that had to have a custom captive page for the public wifi service.

For all the above reasons, I think we can start adding a very simple group functionality now and then over time add more features based on the real needs we have. This way we'll only add features that we really need and avoid creeping in too many features or risk build something unnecessary.
Although instead of adding an arbitrary DB field like "captive_page", I think we should simply provide a way to define data which can be customized depending on the use case.

What we should do in this phase:

• Add a DeviceGroup or Group model and admin (maybe we should call it DeviceGroup to avoid confusion with other models that are called Group, like django's auth.Group model), the model shall have fields like (organization, using OrgMixin), name, description, meta_data, created, modified.
• Add an optional "group" field to Device, the field should be a ForeignKey to DeviceGroup.
• Emit a signal when the group field of a device is change and document the new signal.
• Add the group relation to the DeviceAdmin class, using the django admin autocomplete feature.
• Add a setting which allows to define the schema for DeviceGroup.meta_data and document it.
• Ensure meta_data field is JSON formatted field which will leverage the work we did with JSON schema to show the UI, by default the schema will be empty and allow any property so the user can add what they need.
• Both adminstrators and operators user types shall be able to edit config.Group.
• Add group REST API list and detail endpoints, eg: /api/v1/controller/group/, /api/v1/controller/group/{id}/, authentication and permission checks should be consistent with the REST API endpoints added recently in [feature] REST API for main controller features #379 #386 and the ones being added in [api] Rest API for PKI app #455.