Fix white-space collapsing

[devjones]

The Anki html renderer does not properly collapse white space in the current implementation.

Showdownjs's makeHtml will convert the following string:
This is a really really long sentence

into an html snippet like:

<p>This&nbsp;is&nbsp;a&nbsp;really&nbsp;really&nbsp;long&nbsp;sentence</p>

This normally renders fine, but if you are on a mobile device with limited screen width, the sentence will be broken at a whatever character hits the maximum width of the card. Depending on screen width, you may have the first line break in the middle of a word.

This isn't what we want. We want white space to collapse at the last word that can be fully displayed in a line. The only way I've found to do this is by replacing   with literal spaces. In this case, the returned html snippet will be:

<p>This is a really really long sentence</p>

If the screen width maxes out at the second character in the word long then that entire word will be pushed to the next line.

[putzwasser]

This introduces a new problem: Intentionally added non-breaking spaces get removed/replaced.