Add Credo to the pre-commit

[SimonLab]

see #34

I've also refactor a few part of the code to match Credo style guide.

[codecov-io]

Codecov Report

Merging #44 into master will not change coverage. The diff coverage is 69.23%.

@@         Coverage Diff          @@
##           master   #44   +/-   ##
====================================
  Coverage      82%   82%           
====================================
  Files           8     8           
  Lines          50    50           
====================================
  Hits           41    41           
  Misses          9     9

Impacted Files	Coverage Δ
web/controllers/repo_controller.ex	0% <0%> (ø)	:arrow_up:
web/controllers/webhooks_controller.ex	100% <100%> (ø)	:arrow_up:
web/models/user_from_auth.ex	100% <100%> (ø)	:arrow_up:
web/controllers/auth.ex	85.71% <100%> (ø)	:arrow_up:

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[samhstn]

Looks good 👍