Include warnings-ng reports in templates

[strangelookingnerd]

From personal experience I think it makes sense to include the reports from warnings-ng in the emails send by Jenkins. I know that one can customize the templates as desired but I'd love to see this added as a recommendation.

I added the reports only to the currently documented templates and left the other untouched.
Regarding the code-formatting: It is a mess already and I did not bother - let me know if you want me to change it.

See the screenshots below that demonstrate the feature:

groovy-html.template

groovy-text.template

html.jelly

text.jelly

Testing done

Existing tests run fine. Manually tested visual changes are as intended - see screenshots for details.

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[basil]

Very nice. Thanks for the PR!