Split status checks

[lkysow]

Split single top-level status check into one plan and one apply check.
These checks will also now track the whole pull request status rather
than just the last action.

Fixes #446

[codecov]

Codecov Report

Merging #514 into master will increase coverage by 0.14%.
The diff coverage is 85.05%.

@@            Coverage Diff             @@
##           master     #514      +/-   ##
==========================================
+ Coverage   72.04%   72.19%   +0.14%     
==========================================
  Files          62       62              
  Lines        4465     4503      +38     
==========================================
+ Hits         3217     3251      +34     
- Misses       1005     1010       +5     
+ Partials      243      242       -1

Impacted Files	Coverage Δ
server/events/commit_status_updater.go	100% <100%> (ø)	⬆️
server/events/project_command_runner.go	79.28% <100%> (+0.3%)	⬆️
server/events/command_runner.go	67.12% <70%> (+1.66%)	⬆️
server/events/models/models.go	85.6% <88.46%> (+0.3%)	⬆️
server/events/db/boltdb.go	75.23% <90.9%> (+0.35%)	⬆️

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