Add build button and hide rebuild when it can't work.

[KostyaSha]

Fixes #191

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This change is 

[codecov-io]

Codecov Report

Merging #213 into master will decrease coverage by 1.06%.
The diff coverage is 1.88%.

@@             Coverage Diff              @@
##             master     #213      +/-   ##
============================================
- Coverage     55.65%   54.59%   -1.07%     
+ Complexity      608      604       -4     
============================================
  Files           113      113              
  Lines          2918     2962      +44     
  Branches        232      240       +8     
============================================
- Hits           1624     1617       -7     
- Misses         1203     1256      +53     
+ Partials         91       89       -2

Impacted Files	Coverage Δ	Complexity Δ
...a/github/integration/generic/GitHubRepository.java	68.88% <ø> (ø)	16 <0> (ø)	⬇️
...hub/integration/branch/GitHubBranchRepository.java	7.89% <0%> (-2%)	6 <0> (ø)
...plugins/github/pullrequest/GitHubPRRepository.java	10.34% <3.33%> (-2.29%)	5 <0> (ø)
...est/webhook/GitHubPRRepositoryNameContributor.java	0% <0%> (-80%)	0% <0%> (-2%)
...webhook/GitHubBranchRepositoryNameContributor.java	0% <0%> (-60%)	0% <0%> (-2%)

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