Upgrade to Bootstrap 4

This MR upgrade Bootstrap to version (4.3.1)

Closes #258 (closed)

Review app: http://winh-bootstrap-4.178.62.207.141.xip.io/

Migration bugs

• Feedback section (tabs are touching edges, overall looks odd)

• Versions menu isn't working

• The main container (top-level nav overlays h1) example 1 example 2

• The blue color of the links seems "bluer"

• The spacing between different divs seem to be uneven

• Footnotes look different

• Wrapping inline code blocks in tables looks weird

• On hover over global nav badges to tooltip flickers.

• Underline on hover over feedback tabs

• <hr> missing on the ToC before the "help and feedback"'s <li> (also seems to be misaligned)

• Extra margin on the global nav

• Inline code block highlighting in colored boxes

• Search on 404 page + different blue on the "All results" button + different "Go back" button

• Versions button doesn't expand the menu on click (iPhone 7)

• "On this page" button expands the ToC but starts with 2 columns then goes to one column (iPhone 7)

• Have to check how Disqus is displaying

• Footer content not center aligned.

• In code blocks, when clicking the "copy" button, it should display "copied" (currently highlights the code block)

• bug on the big badges (e.g., http://winh-bootstrap-4.178.62.207.141.xip.io/ee/user/project/operations/feature_flags.html):