Response times scatter plot

[tilln]

The plugin currently publishes response time charts for the build based on the stored sample data, both on the /performance/trendReport and /performance/uriReport page.
These charts are line charts, with each individual sample being plotted and connected with straight lines (first example below).
This is not ideal as it hides significant details (clustering, queuing, gaps, outlier etc.) and would be much better presented as scatter plot, i.e. individual dots without lines in between (second example below).
This PR implements this change, which is purely affecting presentation of the same data.

[codecov]

Codecov Report

Merging #192 into master will decrease coverage by 0.19%.
The diff coverage is 0%.

@@            Coverage Diff            @@
##           master     #192     +/-   ##
=========================================
- Coverage   71.61%   71.42%   -0.2%     
=========================================
  Files          45       45             
  Lines        4245     4248      +3     
  Branches      761      761             
=========================================
- Hits         3040     3034      -6     
- Misses        908      914      +6     
- Partials      297      300      +3

Impacted Files	Coverage Δ
.../performance/actions/PerformanceProjectAction.java	54.08% <0%> (-0.37%)	⬇️
...lugins/performance/build/PerformanceTestBuild.java	70% <0%> (-3%)	⬇️

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