Work around FireFox bugs that affects Utterscroll in <iframe>

Mouseup and mousedown events are simply lost, sometimes, when Utterscroll
runs in an <iframe> and cooperates with Utterscroll in the parent window.
Work aound this by checking the mosue button status, via `event.buttons`.
Return original moseup/down event return values.
Edit debug messages and comments and whitespace.
Add more text to embedded-comments page, so easier to test Utterscroll.
Simplify checkIfMissedMousedown().
Merge branch 'topic-fix-utterscroll-iframe-firefox-issues'.
Make <iframe> Utterscroll compatible with jQuery 2.x

jQuery 2.x might not have $.browser.
Simplify embedded comments API: Create iframe scr attribute via Javascript.
Specify which version of Modernizr to use.
Move /-/res/embedded-comments.js to /-/debiki-embedded-comments.js.
Extract Debiki server origin from debiki-embedded-comments.js script src.
Merge branch 'topic-simplify-iframe-api'.
Show "Loading comments..." message until iframe loaded.
Use a <div> not an <iframe> to indicate where to place embedded comments

Showing an empty <iframe> until the relevant Javascript has run looks ugly.
Remove pointless HTML.
Work around FireFox issue: FF apparently totally ignores hidden iframes

Set height to 0 instead.