Sync BP data with server

[ssrihari]

TODO:

• Finalise data model for blood pressure
• Add model, migrations, same validations as other models
• Include mergeable concern, add mergeable test for BP
• Write specs for blood pressure api
• Implement sync: adding blood pressures, test this
• Implement sync: retrieving blood pressures, test this

Questions:

• Do we compute risk level on mobile, store it, send it to server?
  • Assumption: we'll compute it on mobile, not send it to server

Data Model, API spec

• id: uuid
• systolic: integer
• diastolic: integer
• created_at: timestamp
• updated_at: timestamp
• patient_id: uuid
• all are required