[1637] api/permissions: Allow the root user to query their own permissions

[ssangervasi]

Closes #1637

Code Changes

• Allow the root user to query their own permissions.
• Test fixtures root user!

Steps to Confirm

• Log in as root_user
• The root user doesn't show up in user management (it's not a record)

Pre-Merge Checklist

• All CI Pipelines Succeeded
• Documentation Updated:
  • documentation complete, or draft/outline provided (tag docs-team to complete/review on this branch)
  • documentation issue created (tag docs-team to complete issue separately)
• Issue Requirements are Met
• Relevant Follow-Up Issues Created
• Update CHANGELOG.md

Description Of Changes

The special-casing of root_user goes all the way into fideslib. This solves our login problem, and my change to get_current_user may prove helpful, but this will probably continue to be weird.

root-user-login.mp4