TaskProtocols model

[k1o0]

• ExperimentProtocol model
• Many to many relationship with Session model
• A test for model
• Update filter
• Update serializer
• Create migration
• Create experiment protocol rows from task protocol strings
• Migrate all task protocol strings to experiment protocols field
• Add to the REST register-files endpoint with validation

We will probably always have to support both the task_protocol and experiment protocols fields as cortexlab rigbox isn't going to change, however task protocol could be hidden from the session view...