Merge branch 'markdown_preview_shortcut' into 'master'

Add shortcut to toggle markdown preview

Allow to toggle markdown preview with shortcut `ctrl+shift+p` (`cmd+shift+p` on OSX).

- If the user is focused on a md textarea, `ctrl-shift-p` will show the preview, otherwise native browser event is fired
- The textarea is re-focused after disabling the preview tab
- Results of markdown render is cached for fast toggles

I'm on linux, so if someone can test it on OS X, it would be great!

Closes #14038

See merge request !3111