Fixed bug in allowed_workflows logic

[netguino]

This fixes a bug where it was required to set allow_custom_workflows in order to validate that the workflows were whitelisted.

If the repo is allowed custom workflows and selects one that is not whitelisted server-side we verify that the repo has defined it.

Fixes Issue 1358

[codecov]

Codecov Report

Merging #1377 (4540f18) into master (a95a194) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1377   +/-   ##
=======================================
  Coverage   69.99%   69.99%           
=======================================
  Files          74       74           
  Lines        5575     5576    +1     
=======================================
+ Hits         3902     3903    +1     
  Misses       1310     1310           
  Partials      363      363           

Impacted Files	Coverage Δ
server/events/yaml/valid/global_cfg.go	89.72% <100.00%> (+0.07%)	⬆️

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