Debounce JenkinsCheck on the number of job failures

[JeanFred]

• Add field consecutive_failures to StatusCheckResult model (and associated migration).
• Retrieve from Jenkins the last good build, and compute from that the number of consecutive failures

5ac9c64 is a first stab with no migration, but severe limitations.

Closes #536

[codecov-io]

Codecov Report

Merging #540 into master will increase coverage by 0.14%. The diff coverage is 93.33%.

@@            Coverage Diff             @@
##           master     #540      +/-   ##
==========================================
+ Coverage   80.87%   81.01%   +0.14%     
==========================================
  Files          45       45              
  Lines        2907     2918      +11     
  Branches      176      177       +1     
==========================================
+ Hits         2351     2364      +13     
+ Misses        497      496       -1     
+ Partials       59       58       -1

Impacted Files	Coverage Δ
cabot/cabotapp/models/base.py	79.52% <100%> (+0.11%)	:arrow_up:
cabot/cabotapp/jenkins.py	97.14% <100%> (+6.82%)	:arrow_up:
cabot/cabotapp/models/jenkins_check_plugin.py	66.66% <83.33%> (+2.51%)	:arrow_up:

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[JeanFred]

Putting that on hold to get #541 in first.