Content management systems, such as the one I'm using to write this piece right now, automatically edit out the extra space at the end of a sentence even if you try to put one in.

No, html ignores extra spaces.   To get the effect of two spaces you need to add a non-breaking space between sentences.   Otherwise html just shows one space.   Actually, it's slightly more complicated.   Because of how browsers handle html, to get the right effect you need to add a space, a non-breaking space and another space.   Otherwise you get weird effects.