Fix broken JS in xhtml

[selfthinker]

When serving DokuWiki as XHTML there are some JavaScript errors. This fixes most of those errors mentioned in FS#2550.

I'll explain the issues further in the comments below.

[selfthinker]

Re: the revisionBoxHandler, the issue was only with .attr('disabled', false) but I rewrote more bits that don't have anything to do with the initial issue.

Re: the footnoteDisplay, the issue was that the content that was put back via .html() was not stripped from the <sup> etc because that element included the xmlns attribute. E.g. the content of the footnote from the wiki:syntax page was this:

<sup xmlns="http://www.w3.org/1999/xhtml"><a href="#fnt__1" id="insitu__fn__1 class="fn_bot">1)</a></sup> 
This is a footnote

And using that in .html() would throw "SyntaxError: An invalid or illegal string was specified". Officially, .html() does not work with XHTML ("This method is not available on XML documents.") but it is otherwise working. I'm not sure but I guess it's the xmlns because XML documents might break if any snippets re-defining the namespace would arbitrarily be put anywhere willy-nilly?

[selfthinker]

Could we ever have block elements inside a footnote? If yes, then this should rather be a <div>.

[splitbrain]

Yes, I think we could.

[selfthinker]

I changed it to a div now.

I have to say it's difficult to restrain myself not improving the semantics here. But that makes more sense after the release (and in a different PR).