[#151] Update highlight.js to version 8.2

Update javascript library highlight.js to latest version 8.2 from
https://highlightjs.org/.
added patch to markdown editor
added links to reveal.js 2.6.2
Merge branch 'feature/update-highlightjs-151' into develop
Merge pull request #180 from hicknhack-software/feature/Improve_Scroll_Behavior

Fix scroll behavior while using keyboard to scroll in text editor

Adds work-around for QTBUG-8682
removed unintentional changes
Merge pull request #179 from hicknhack-software/feature/update_reveal.js_to_2.6.2

Updated reveal.js to 2.6.2
Added Gitter badge
Merge pull request #182 from gitter-badger/gitter-badge

Add a Gitter chat badge to README.md
Move link to gitter down and add mailing list link
merge upstream changes