HTTPS everything

WHEW! Big diff... @clouserw, this one's for you! 

- changed all http:// to https:// wherever possible.
- removed /en-us/ from mozilla.org links wherever possible so it redirects to correct/best locale.
- fixed some invalid Markdown markup.
- fixed a handful of broken links.
- used shorthand Markdown notation for links and mailto: URLs to reduce duplication and chance for errors.

Everything I grepped looked solid. Everything passed my local link checker (with the exception of the relative marketplace links). Everything looked good in the 9000 page diff below (comparing side-by-side is easier, but still a lot of minor tweaks).

:metal: