Quotes and m-dashes are not properly escaped in epub

Subject: missing escapes in epub builder

Problem

• I'm trying to go from sphinx to mobi for Kindle. I'm using the epub builder with the epub theme on sphinx 1.8.3 and then passing to kindlegen
• Most stuff works but all the "smart quotes" and m-dashes and whatnot are coming out as goofy characters in mobi, like ’ instead of an apostrophe and – instead of m-dash.
• If I turn off smartquotes the apostrophe's work but not the m-dashes.

The rst looks like:

*****                                                                                                                                                                                            
Intro
*****
Computers are everywhere. They're in our purses, our offices, our TV's, our hobby-dens,
our cars, and our toasters. Many of us spend significant parts of our days operating these

The xhtml source for the epub looks like this:

<h1>Intro</h1>
<p>Computers are everywhere. They’re in our purses, our offices, our TV’s, our hobby-dens,
our cars, and our toasters. Many of us spend significant parts of our days operating these

Procedure to reproduce the problem

make epub

# epub viewers sometimes show these characters ok, but once 
# you run kindlegen they go to the weird chars
kindlegen _build/epub/book.epub

Expected results

I'd expect it to be They’re in our purses instead of the special apostrophe or something like that in the xhtml source on the way into the epub.

Environment info

• OS: Ubuntu 18.10
• Python version: 3.6.7
• Sphinx version: 1.8.3
• KindleGen v2.9 for Linux 2.6 i386

Reproduce the issue with the mentioned epub builder using make epub, then pass _build/epub/book.epub to kindlegen and inspect the generated XHTML. Trace the epub builder's character handling and add a regression check for smart quotes and m-dashes; done means KindleGen no longer produces mojibake.