Unique names per institution

[dandavies99]

Currently, models that inherit from BaseModelMandatoryName must have a unique name. Because of the object-level permission setup, the situation could arise that a user cannot name an object of one of those models what they want because a user in another institution has already created an object with that name.

This many not be much of a problem in practice, but if it does become one, it can be resolved by applying the per-institution unique constraint to those models that is used in the clean method of the Experiment model.

Aha! Link: https://imperial-college-ict.aha.io/requirements/RSE-138-1