Make policy checks its own apply requirement. (#61)

[nishkrishnan]

This makes policy checking a better experience for the customer and operator:

• Mergeability is decoupled from policy checking. There are a bunch of issues I've seen in the issues about mergeability not working for their usecase.
• The message is updated to be explicit about addressing failed policies.
• policy checking now failures only block the project that it failed for and not every project in the PR.
• The apply requirement gets loaded when a server has policy checking enabled

[codecov]

Codecov Report

Merging #1499 (0c75cc3) into master (921ebbc) will decrease coverage by 0.15%.
The diff coverage is 59.72%.

@@            Coverage Diff             @@
##           master    #1499      +/-   ##
==========================================
- Coverage   70.23%   70.08%   -0.16%     
==========================================
  Files          94       94              
  Lines        6441     6485      +44     
==========================================
+ Hits         4524     4545      +21     
- Misses       1533     1554      +21     
- Partials      384      386       +2     

Impacted Files	Coverage Δ
server/events/apply_command_runner.go	71.60% <ø> (-2.26%)	⬇️
server/events/comment_parser.go	94.20% <ø> (ø)
server/events/models/models.go	74.10% <ø> (ø)
server/events/project_command_runner.go	46.66% <0.00%> (-0.87%)	⬇️
server/events/yaml/raw/global_cfg.go	0.00% <0.00%> (ø)
server/events/plan_command_runner.go	40.94% <50.00%> (+0.14%)	⬆️
server/events/yaml/valid/global_cfg.go	85.45% <52.94%> (-4.16%)	⬇️
server/events/command_runner.go	63.07% <78.94%> (-0.86%)	⬇️
server/events/project_command_context_builder.go	97.54% <100.00%> (+2.85%)	⬆️
server/server.go	65.14% <100.00%> (+0.56%)	⬆️
... and 1 more

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[msarvar]

LBTM 🎉