feat: Adds a safeguard check of permissions for modifications of repos/guidelines

[frgfm]

This PR introduces the following modifications:

• adds a safeguard checking the GitHub permission of the user for edit operations on repo & guidelines
• updates e2e and unit tests
• updates README

[codecov]

Codecov Report

Attention: 10 lines in your changes are missing coverage. Please review.

Comparison is base (625787b) 85.65% compared to head (02cb0da) 86.25%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #31      +/-   ##
==========================================
+ Coverage   85.65%   86.25%   +0.60%     
==========================================
  Files          27       27              
  Lines         725      764      +39     
==========================================
+ Hits          621      659      +38     
- Misses        104      105       +1     

Flag	Coverage Δ
unittests	86.25% <84.12%> (+0.60%)	⬆️

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Files	Coverage Δ
src/app/crud/crud_guideline.py	100.00% <100.00%> (ø)
src/app/models.py	100.00% <100.00%> (ø)
src/app/schemas/base.py	100.00% <100.00%> (ø)
src/app/schemas/guidelines.py	100.00% <100.00%> (ø)
src/app/schemas/repos.py	100.00% <100.00%> (ø)
src/app/schemas/users.py	100.00% <100.00%> (ø)
src/app/api/api_v1/endpoints/repos.py	94.91% <87.50%> (+6.45%)	⬆️
src/app/api/api_v1/endpoints/users.py	97.14% <85.71%> (+0.26%)	⬆️
src/app/api/api_v1/endpoints/login.py	62.22% <0.00%> (-2.90%)	⬇️
src/app/services/github.py	89.13% <71.42%> (+6.20%)	⬆️
... and 1 more

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