Add automerge capability

[lkysow]

Automerge merges pull requests if all plans have been applied successfully.

Can be enabled via the --automerge flag or a top-level automerge: true key in atlantis.yaml.

Notes:

• Builds on great work from @brndnmtthws
• When automerge is enabled, all plans must succeed for any plans to be saved
• Switches to golangci-lint from gometalinter
• Stores PR status in local boltDB database
• Closes Automatically merge when all apply's complete #186
• Closes Implement an automerge feature. #389

[codecov]

Codecov Report

Merging #457 into master will decrease coverage by 0.24%.
The diff coverage is 70.71%.

@@            Coverage Diff             @@
##           master     #457      +/-   ##
==========================================
- Coverage   70.78%   70.53%   -0.25%     
==========================================
  Files          63       63              
  Lines        3987     4243     +256     
==========================================
+ Hits         2822     2993     +171     
- Misses        970     1028      +58     
- Partials      195      222      +27

Impacted Files	Coverage Δ
server/events/yaml/valid/valid.go	93.54% <ø> (ø)	⬆️
server/events/project_command_builder.go	84.47% <ø> (ø)	⬆️
cmd/server.go	78.94% <ø> (ø)	⬆️
server/events/vcs/common/common.go	100% <ø> (ø)
...r/events/vcs/bitbucketserver/request_validation.go	58.82% <ø> (ø)	⬆️
server/user_config.go	100% <ø> (ø)	⬆️
testdrive/utils.go	0% <0%> (ø)	⬆️
server/locks_controller.go	87.14% <0%> (-2.57%)	⬇️
server/events/vcs/proxy.go	28.57% <0%> (-2.2%)	⬇️
server/events/pull_closed_executor.go	89.74% <0%> (-4.86%)	⬇️
... and 17 more

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