fix: update permissions on classes with missing / unclear permissions

[matthewelwell]

Changes

1. Adds the required permission class to the SlackGetChannelsViewSet
2. Moves the staff_member_required from urls.py to decorate the methods on the class for consistency with other permission definitions

How did you test this code?

Added new tests for permissions failure use cases. Existing tests cover happy paths.

[vercel]

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3 Skipped Deployments

Name	Status	Preview	Comments	Updated (UTC)
docs	⬜️ Ignored (Inspect)	Visit Preview		Sep 30, 2024 9:27am
flagsmith-frontend-preview	⬜️ Ignored (Inspect)	Visit Preview		Sep 30, 2024 9:27am
flagsmith-frontend-staging	⬜️ Ignored (Inspect)	Visit Preview		Sep 30, 2024 9:27am

[github-actions]

Docker builds report

Image	Build Status	Security report
ghcr.io/flagsmith/flagsmith-api-test:pr-4667	Finished ✅	Skipped
ghcr.io/flagsmith/flagsmith-e2e:pr-4667	Finished ✅	Skipped
ghcr.io/flagsmith/flagsmith-frontend:pr-4667	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith-api:pr-4667	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith:pr-4667	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith-private-cloud:pr-4667	Finished ✅	Results ✅

[github-actions]

Uffizzi Preview deployment-56707 was deleted.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.19%. Comparing base (dd1dd32) to head (0f8bf10).
Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4667   +/-   ##
=======================================
  Coverage   97.18%   97.19%           
=======================================
  Files        1166     1166           
  Lines       40369    40405   +36     
=======================================
+ Hits        39234    39270   +36     
  Misses       1135     1135           

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