Add support for organisations and user groups

[pmslavin]

This issue is to add the concept of an 'organisation' to Hydra, that is a namespace which encapsulates a set of entities and provides a context within which operations such as sharing, permissions, and user/resource lookups can be performed for those entities.

This should be achieved by identifying a set of common workflows, producing tests which replicate these, and then ensuring that Hydra provides an interface which allows these to be performed in a way that is resilient to errors.

[knoxsp]

Based on some feedback, we have decided that the 'organisation' table should be removed. User groups should exist within a project, but only a top-level project: i.e. a project which has no parents.