Bitbucket server aka stash

[lkysow]

Fixes #190

Todo

• Support webhook secrets
• Test
• Update docs

Note: Triggering plan on pull request updates does not work because Bitbucket Server doesn't send an event when the pull request is updated (although it does send an event when any ref in the repo is updated). I've created #198 to track this.

[codecov]

Codecov Report

Merging #194 into master will decrease coverage by 1.66%.
The diff coverage is 44.08%.

@@            Coverage Diff             @@
##           master     #194      +/-   ##
==========================================
- Coverage   71.05%   69.38%   -1.67%     
==========================================
  Files          54       56       +2     
  Lines        2843     3126     +283     
==========================================
+ Hits         2020     2169     +149     
- Misses        701      807     +106     
- Partials      122      150      +28

Impacted Files	Coverage Δ
server/events/vcs/proxy.go	0% <0%> (ø)	⬆️
server/events/command_runner.go	68.84% <0%> (-3.76%)	⬇️
server/locks_controller.go	92.3% <0%> (-3.08%)	⬇️
server/events/models/models.go	82.35% <100%> (+25.21%)	⬆️
server/events_controller.go	52.86% <2.98%> (-13.44%)	⬇️
server/events/vcs/bitbucketcloud/client.go	39.21% <30.76%> (ø)
server/events/vcs/bitbucketserver/client.go	38.09% <38.09%> (ø)
server/server.go	62.13% <41.93%> (-1.66%)	⬇️
...r/events/vcs/bitbucketserver/request_validation.go	58.82% <58.82%> (ø)
cmd/server.go	79.02% <64.28%> (-0.4%)	⬇️
... and 4 more

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