Truncate long text fields

[mrginglymus]

Implementation of jenkinsci/checks-api-plugin#68

[codecov]

Codecov Report

Merging #101 (6397e47) into master (70de35e) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##             master     #101   +/-   ##
=========================================
  Coverage     82.37%   82.37%           
  Complexity      148      148           
=========================================
  Files            13       13           
  Lines           437      437           
  Branches         41       41           
=========================================
  Hits            360      360           
  Misses           61       61           
  Partials         16       16           

Impacted Files	Coverage Δ	Complexity Δ
...ins/plugins/checks/github/GitHubChecksDetails.java	76.34% <100.00%> (ø)	24.00 <0.00> (ø)

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