Headings hierarchy is broken when using Bootstrap .modal-title and .panel-title

AFFECTS:

• All pages
• xwiki:XWiki.WatchListRss
• xwiki:Main.WebHome
• etc.

ERROR:

• org.xwiki.test.webstandards.CustomDutchWebGuidelinesValidationTest.Validating HTML5 Dutch Web Guidelines validity for: xwiki:XWiki.WatchListRss executed with credentials Admin:admin (from org.xwiki.test.webstandards.AllTests)
• ERROR: The header hierarchy seems to have been skipped somewhere in the document (example: skips from <h1> to <h3> without a <h2> header. This might mean that headings have been used for their size instead of their place within the document structure.

INVALIDATES:

• Dutch Guidelines: validateRpd3s3()
  • https://github.com/xwiki/xwiki-platform/blob/master/xwiki-platform-tools/xwiki-platform-tool-standards-validator/src/main/java/org/xwiki/validator/DutchWebGuidelinesValidator.java#L433-L449
• R-pd.3.3 "Do not skip any levels in the hierarchy of headings in the markup."

EXPLANATION:

• Bootstrap modals are using H4 for the styling of modal-titles, see https://getbootstrap.com/docs/3.3/javascript/#modals-examples
• Bootstrap panels are using H3 when defining panel-titles (but they overwrite the font-size manually, see https://getbootstrap.com/docs/3.3/components/#panels
• #exportModal, see https://github.com/xwiki/xwiki-platform/blob/master/xwiki-platform-core/xwiki-platform-flamingo/xwiki-platform-flamingo-skin/xwiki-platform-flamingo-skin-resources/src/main/resources/flamingo/export_modal.vm

FIXES:

• Remove the H4 usage for .modal-title and add styling
  • verify other places where we have Bootstrap modals, see https://design.xwiki.org/xwiki/bin/view/Proposal/Polishing10x/PolishingModals/#BootstrapModal (12 places)
• Remove the H3 usage for .panel-title (styling is already there)
• The H3-H4 will be replaced with DIVs
• Other links:
  • https://www.w3.org/TR/WCAG20-TECHS/G141.html
  • https://www.w3.org/TR/WCAG20-TECHS/H69.html