User management

Hilary counterpart of https://github.com/oaeproject/3akai-ux/pull/3698

Introduces REST endpoints for:

- Checking if a username is already taken through the global tenant
- api/download/signed through the global tenant for profile pictures
- Getting a user's basic profile information through the global tenant
- Updating a user's basic profile information through the global tenant
- Creating a new user with local auth strategy through the global tenant

Tests are coming up.