Benchmark report menu should have a scrollbar if (and only if) the browser window's height is too small to show it all

To reproduce:

• Adjust nurseRosteringSprintBenchmarkConfig.xml to run 20 solvers and on all 20 problems
• Run NurseRosteringBenchmarkApp with the program argument "sprint"
• Open local/data/nurserostering/sprint/.../index.html
• Adjust the window size of your browser to a lower height. Notice how it's impossible to see or click on the last menu items (solver benchmarks, benchmark information etc)

To fix it, adjust benchmarkReport.html.ftl accordingly.
https://github.com/droolsjbpm/optaplanner/blob/master/optaplanner-benchmark/src/main/resources/org/optaplanner/benchmark/impl/report/benchmarkReport.html.ftl