Remove Guest as an assignable role

[andreasfrisch]

🌟 Changes:

• Filter out 'Guest' role from list of assignable roles (we still want the Guest role in the system, it is just not assignable).
• Fail requests asking to assign the Guest role (should people attempt to use the API directly).
• When checking permissions, always include the guest permissions (cached at the request level)

[DFDS-Snyk]

✅ Snyk checks have passed. No issues have been found so far.

Status	Scan Engine	Critical	High	Medium	Low	Total (0)
✅	Open Source Security	0	0	0	0	0 issues
✅	Licenses	0	0	0	0	0 issues
✅	Code Security	0	0	0	0	0 issues

💻 Catch issues earlier using the plugins for VS Code, JetBrains IDEs, Visual Studio, and Eclipse.