Make test result labels more stylable

[noomorph]

Context

Adding a custom stylesheet to a generated Allure Report is not too complicated, especially if one uses plugins.

What is much more challenging, however, is to customize the core sections of the report, because they don't include enough HTML markup to target with custom CSS rules.

This pull request aims to standardize CSS class names across the Test Result section:

• .pane__section-title – now used in each pane__section.
• .pane__section-content – a new CSS class which wraps in a generic way all the content inside those panes: severity, categories, tags, owner, duration, etc.

Markup	Screenshot

Note

There is a minor change to conciliate h3.pane__section-title and span.pane__section-title – the inline titles in span will have ::after pseudo-element with a colon. This way we ensure that the colon is easy to add or remove in custom CSS styles.

Checklist

• Sign Allure CLA
• Provide unit tests (not applicable to Handlebars changes, I guess)
• Check manually that the generated report has no visual regressions

[A1exKH]

LGTM.