[keyserver] Add doNotRegister flag to siwe_auth endpoint
Summary: We need this optional flag so that we can prevent the web app from registering new accounts. See ENG-2930 for details.
Test Plan:
- Delete the Comm account for my test SIWE user
- Patch web/account/siwe-login-form.react.js to include this flag
- Confirm that it's now impossible to SIWE with my test SIWE user, and that the correct error is displayed in the console
Reviewers: atul, ginsu
Reviewed By: ginsu
Subscribers: tomek
Differential Revision: https://phab.comm.dev/D8753