Policy: Use no_one rather than false

[MrSerth]

In CodeOcean, we have a no_one policy that denies access to some actions. In CodeHarbor, we didn't add the policy when introducing Pundit, since it wasn't yet used. However, with #1230, we'll do so (and already did implicitly). Hence, we are adding the method here already and use it where appropriate

//cc @Melhaya for the boy scouting rule

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.45%. Comparing base (4d760dd) to head (96528a3).
Report is 5 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1524   +/-   ##
=======================================
  Coverage   93.44%   93.45%           
=======================================
  Files         115      115           
  Lines        2808     2810    +2     
=======================================
+ Hits         2624     2626    +2     
  Misses        184      184           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.