Add public_user_uuid field to User model

[tompollard]

It would be helpful to have a unique field to identify user accounts. One use case for this ID is for our project to allow PhysioNet users to login to the NSTRI platform (see: #2383).

The unique ID would allow the NSTRI platform to maintain a persistent link between logins and PhysioNet users (see: #2383).

PhysioNet users can update their email, username, etc, so these fields do not meet this need. It seems weird to be sharing the primary key in our database, so instead we are creating a new public_user_uuid field.

This pull request:

• Adds a public_user_uuid to the User model
• Adds a 3-step migration:
  1. create the field, first nullable;
  2. backfill existing user data
  3. add the non-null constraint.

[Chrystinne]

@tompollard The code looks good to me!