Introduce new mergeable requirement

[lkysow]

Introduce new mergeable requirement, in similar vein to the approved
requirement.

Fixes #43.

Notes:

• Refines work from Introduce new mergeable requirement #385 by also checking mergeable status from github, checking required approvals from gitlab and refining documentation

[codecov]

Codecov Report

Merging #388 into master will decrease coverage by 0.6%.
The diff coverage is 36.23%.

@@            Coverage Diff             @@
##           master     #388      +/-   ##
==========================================
- Coverage   70.37%   69.77%   -0.61%     
==========================================
  Files          62       62              
  Lines        3743     3801      +58     
==========================================
+ Hits         2634     2652      +18     
- Misses        922      960      +38     
- Partials      187      189       +2

Impacted Files	Coverage Δ
cmd/server.go	79.16% <ø> (ø)	⬆️
testdrive/github.go	0% <ø> (ø)	⬆️
server/events/vcs/bitbucketcloud/client.go	41.02% <0%> (-5.13%)	⬇️
server/events/vcs/proxy.go	30.76% <0%> (-2.57%)	⬇️
server/events/vcs/not_configured_vcs_client.go	0% <0%> (ø)	⬆️
server/events/vcs/gitlab_client.go	19.81% <0%> (-1.41%)	⬇️
server/events/vcs/bitbucketserver/client.go	32% <0%> (-3.83%)	⬇️
server/events/yaml/raw/project.go	100% <100%> (ø)	⬆️
server/server.go	66.27% <100%> (+0.26%)	⬆️
server/events/vcs/github_client.go	67.94% <75%> (+5.09%)	⬆️
... and 2 more

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