Refactor sessions to be session instances

[amberchow8]

• Create model for Session in api/src/models/study.ts
• Modify sessionSchema to be customizedSessionSchema in api/src/models/study.ts (follow CustomizedBattery)
• Modify variantSchema so that it contains a list of Session object ids (that point to a CustomizedSession)

[arath7]

make a new study, look at mongodb and see if the structure is what you expect

instead of editing the endpoint, edit the variantSchema