Support pre-workflow hooks on all comment/auto triggered commands. (#43)

[nishkrishnan]

Allows you to actually do dynamic configuration generation since unlock command can be run at anytime which would then nuke the working directory and the PR would need to be opened again from scratch.

[codecov]

Codecov Report

Merging #1418 (7b58de5) into master (5e4bde4) will increase coverage by 0.24%.
The diff coverage is 92.59%.

@@            Coverage Diff             @@
##           master    #1418      +/-   ##
==========================================
+ Coverage   69.40%   69.64%   +0.24%     
==========================================
  Files          93       93              
  Lines        6291     6280      -11     
==========================================
+ Hits         4366     4374       +8     
+ Misses       1543     1529      -14     
+ Partials      382      377       -5     

Impacted Files	Coverage Δ
server/events/runtime/pre_workflow_hook_runner.go	100.00% <ø> (ø)
server/events_controller.go	56.07% <ø> (-0.32%)	⬇️
server/events/command_runner.go	66.66% <50.00%> (+1.66%)	⬆️
server/events/delete_lock_command.go	75.00% <100.00%> (+1.92%)	⬆️
server/events/pre_workflow_hooks_command_runner.go	100.00% <100.00%> (+35.41%)	⬆️
server/events/yaml/valid/global_cfg.go	89.33% <100.00%> (+0.07%)	⬆️
server/server.go	64.91% <100.00%> (-0.15%)	⬇️

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[msarvar]

This should fix following two bugs

1. Fixes pre-workflow-hooks not logging errors Atlantis doesn't log pre_workflow_hooks #1371
2. Prevents pre-workflow-hook from locking and cloning the dir if there are no hooks registered. PreWorkflowHook do not respect SkipCloneNoChanges #1342

[acastle]

LGTM 🚀