InstrumentSessionView should return a Visit object

[DominicOram]

Currently the instrumentSession provided by the component made in https://github.com/DiamondLightSource/atlas/pull/129/changes gives a string of the full session. However, it is often useful for queries to break this into proposal/sessionNumber (https://github.com/DiamondLightSource/atlas/pull/150/changes/BASE..db987190745743f998993c53c0261e472e5699dc#r3748402923). We already have a Visit object that holds the session split into parts, we should pass this around instead of the raw string

Acceptance Criteria

• A new fullStringRepresentation field is added to Visit
• The instrumentSession returns a Visit
• Uses of the instrumentSession grab out of the Visit the data they need
• We rename Visit to Session to maintain consistency?

[EmsArnold]

• We rename Visit to Session to maintain consistency?

I think this should follow once the component is renamed (sci-react-ui #93)