Support bitbucket server pr:deleted event

[lkysow]

If a pull request is deleted, Atlantis should delete the locks.
Fixes #498

[codecov]

Codecov Report

Merging #510 into master will decrease coverage by 1.66%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master    #510      +/-   ##
=========================================
- Coverage   71.57%   69.9%   -1.67%     
=========================================
  Files          62      62              
  Lines        4464    4466       +2     
=========================================
- Hits         3195    3122      -73     
- Misses       1031    1106      +75     
  Partials      238     238

Impacted Files	Coverage Δ
server/events_controller.go	62.12% <100%> (+8.93%)	⬆️
server/events/event_parser.go	87.97% <100%> (ø)	⬆️
server/logging/simple_logger.go	88.88% <100%> (+0.13%)	⬆️
server/events/runtime/apply_step_runner.go	38.18% <0%> (-48.19%)	⬇️
server/events/terraform/terraform_client.go	47.15% <0%> (-31.71%)	⬇️
server/events/vcs/bitbucketserver/client.go	33.33% <0%> (-1.97%)	⬇️

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