Get mergeable status *before* changing build status

[lkysow]

Small refactor of changes from @pratikmallya via #454 (thanks!).

Get the Mergeable status of the PR before updating the
Status. This allows for the Status to be a required
Status for merging the PR. This allows us to use the
atlantis status as a Required status, which is the
configuration often used, since most deployments
want to ensure tf plans have all been successfully
applied before merging

Closes #453
Closes #454

[codecov]

Codecov Report

Merging #469 into master will decrease coverage by 0.02%.
The diff coverage is 58.33%.

@@            Coverage Diff             @@
##           master     #469      +/-   ##
==========================================
- Coverage   70.57%   70.55%   -0.03%     
==========================================
  Files          63       63              
  Lines        4241     4248       +7     
==========================================
+ Hits         2993     2997       +4     
- Misses       1027     1029       +2     
- Partials      221      222       +1

Impacted Files	Coverage Δ
server/server.go	67.29% <ø> (-0.13%)	⬇️
server/events/models/models.go	85.26% <ø> (ø)	⬆️
server/events/project_command_builder.go	84.68% <100%> (+0.2%)	⬆️
server/events/project_command_runner.go	78.98% <33.33%> (+0.97%)	⬆️
server/events/command_runner.go	65.46% <50%> (-0.5%)	⬇️
server/events/vcs/github_client.go	70.96% <0%> (-2.43%)	⬇️
server/events/vcs/bitbucketcloud/client.go	39.66% <0%> (-0.5%)	⬇️
server/events/vcs/bitbucketserver/client.go	30.18% <0%> (-0.44%)	⬇️
server/events/terraform/terraform_client.go	75.26% <0%> (+1.29%)	⬆️

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