Fix strikethrough rendering in markdown converter

Previously the <del> tag that Discount emits would be removed by Qt since it's not in the subset of HTML that it understands. It does know about <s> though, so replace <del> with <s> to render strikethroughs correctly.

Note that I couldn't get Okular to build locally (PEBCAK issue) so I split out this file to debug it. I've tested against http://daringfireball.net/projects/downloads/MarkdownTest_1.0.zip and https://github.com/Orc/discount/tree/ff866f73d4b45c1a8581fc7c81dd2f568aefdfd8/tests/data and the only difference is that trailing whitespace is now trimmed from the end of a line:

8c8
< <p style=" margin-top:12px; margin-bottom:12px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;">Dashes:</p>
---
> <p style=" margin-top:12px; margin-bottom:12px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;">Dashes: </p>

A quick search suggests this isn't a problem though https://stackoverflow.com/a/27026403:

Within block tags (i.e. <p>, <h1>, <div>, ...) spaces as well as line breaks at the beginning or end of the tags should always be ignored (i.e. <p>test</p> should look the same as <p> test </p>).

I also tried https://doc.qt.io/qt-5/qtextdocument.html#setMarkdown which solves the strikethrough problem, though there are many visible differences with that approach. Most notably, everything is squashed together and code blocks lose their formatting.