Auth as app

[Marco Franssen (marcofranssen)]

This PR adds authenticating tabia as a Github App.

Current implementation will limit the GitHub App authentication to a single organization, whereas the token based auth allows to fetch information from multiple organizations at a time.

[codecov]

Codecov Report

Merging #88 (a34b1d0) into develop (b39b7a5) will decrease coverage by 0.45%.
The diff coverage is 60.00%.

@@             Coverage Diff             @@
##           develop      #88      +/-   ##
===========================================
- Coverage    66.66%   66.21%   -0.46%     
===========================================
  Files           17       18       +1     
  Lines          408      438      +30     
===========================================
+ Hits           272      290      +18     
- Misses         117      129      +12     
  Partials        19       19              

Flag	Coverage Δ
unittests	66.21% <60.00%> (-0.46%)	⬇️

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Impacted Files	Coverage Δ
lib/github/app.go	60.00% <60.00%> (ø)

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