Add flycheck-cask to the manual
Update extension URLs
Update Cask URLs and remove all Carton references
Credit @tom-tan for his extension
Pseudo-bootstrappify the manual :)

I'd like to use Bootstrap directly, but getting Texinfo to output HTML 5
with the right elements for Bootstrap is a major PITA, and requires
advanced Perl wizardry.  That's definitely beyond my limits, so I just
copied the corresponding styles from the Bootstrap stylesheets.  The
results are not ideal, but quite ok.
Add style for blockquotes
Add licensing information to the HTML frontmatter