#726 Report license header check issues at the end of the build

• License header check changes:
  • Renamed license-header-list.txt to license-header-issues.txt.
  • Ignore .mvn/.develocity/develocity-workspace-id file not having a license header.
• Problem reporter changes:
  • Renamed Problem(s) to FileProblem(s), to make room for problems not reported on files.
  • Added BuildProblems abstract class for different kinds of build problems.
  • Build collectors now return their own problems, allowing different collectors to return different kinds of problems.
  • Defined getCount commonly, in BuildProblems, to be able to count over all different kinds of problems.
  • Build problems get a name, to keep different problems apart and be able to report them differently.
  • Build reporter can now handle different kinds of build problems.
  • Various changes to the build problems report, to account for different kinds of build problems.
  • Besides file-based problems, there are now also global problems.
  • Build problems report now has a summary, reporting the status and number of problems reporter by different collectors.
  • Some problem report CSS improvements/fixes.
• Report license header check problems at end of build changes:
  • Jenkinsfile: don't immediately stop build on license header issues.
  • Added license header check problems collector, which collects global problems.

Addresses #726