fix: Forbid admin route for non admin user

[divs4debu]

Fixes #4639

Short description of what this resolves:

Changes proposed in this pull request:

Checklist

• I have read the Contribution & Best practices Guide.
• My branch is up-to-date with the Upstream development branch.
• The acceptance, integration, unit tests and linter pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)

[vercel]

This pull request is being automatically deployed with Vercel (learn more).
To see the status of your deployment, click below or on the icon next to each commit.

🔍 Inspect: https://vercel.com/eventyay/open-event-frontend/8hi7zi6fa
✅ Preview: https://open-event-frontend-git-fork-divs4debu-adminsettings.eventyay.vercel.app

[codecov]

Codecov Report

Merging #4691 into development will increase coverage by 0.84%.
The diff coverage is 66.66%.

@@               Coverage Diff               @@
##           development    #4691      +/-   ##
===============================================
+ Coverage        22.27%   23.11%   +0.84%     
===============================================
  Files              474      476       +2     
  Lines             4988     4975      -13     
  Branches             8        8              
===============================================
+ Hits              1111     1150      +39     
+ Misses            3876     3824      -52     
  Partials             1        1              

Impacted Files	Coverage Δ
app/routes/admin.js	50.00% <66.66%> (+50.00%)	⬆️
app/models/session.js	10.00% <0.00%> (-1.12%)	⬇️
app/router.js	100.00% <0.00%> (ø)
app/routes/admin/sessions.js	0.00% <0.00%> (ø)
app/controllers/events/view.js	0.00% <0.00%> (ø)
app/routes/admin/sessions/list.js	0.00% <0.00%> (ø)
app/controllers/admin/events/list.js	0.00% <0.00%> (ø)
app/controllers/events/view/sessions.js	0.00% <0.00%> (ø)
app/components/public/call-for-speakers.js	0.00% <0.00%> (ø)
app/controllers/events/view/sessions/list.js	0.00% <0.00%> (ø)
... and 26 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 3ba9b1a...cce3740. Read the comment docs.