Separate internal `user_id` from user account information used in OPAQUE.

[indomitableSwan]

In the current implementation, we conflated the user account information used in OPAQUE (normally, e.g., a user email or other memorable account information) with an internal, universally unique identifier for the user (user_id).

We fixed this conflation problem in the specification; for more details see:

• the new version of registration sessions here
• a diff of relevant changes here.

This epic will be complete when the implementation reflects these changes, which includes:

• [x] refactoring registration
• [x] refactoring authentication
• [ ] confirming that any impacted WIP has been updated appropriately.

[marsella]

Some preliminary work is on the 131-register-cryptography branch, which makes the two different types but doesn't start to define where they are each used and created.

[marsella]

I am not confident about separating this work into multiple issues. I think it might have to be one big PR. I will make sub-issues for now but I'm afraid they will end up all being done in one.

[indomitableSwan]

I am not confident about separating this work into multiple issues. I think it might have to be one big PR. I will make sub-issues for now but I'm afraid they will end up all being done in one.

I wouldn't work too hard on making separate issues in that case.