feat: Adds permission check shortcut using repo.installed_by

[frgfm]

This PR adds a way to check permission using installed_by (because it has already been approved).

[codecov]

Codecov Report

Attention: 3 lines in your changes are missing coverage. Please review.

Comparison is base (b7efdf0) 86.42% compared to head (0057e32) 86.42%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #33   +/-   ##
=======================================
  Coverage   86.42%   86.42%           
=======================================
  Files          27       27           
  Lines         766      766           
=======================================
  Hits          662      662           
  Misses        104      104           

Flag	Coverage Δ
unittests	86.42% <62.50%> (ø)

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Files	Coverage Δ
src/app/services/github.py	89.13% <100.00%> (ø)
src/app/api/api_v1/endpoints/repos.py	94.91% <66.66%> (ø)
src/app/api/api_v1/endpoints/guidelines.py	82.97% <50.00%> (ø)

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