When Python Markdown renders an e-mail address, it escapes each

character in order to make it harder for addresses to be scraped. This

can generate some bad entities when adding backslash escaping at the end

of the e-mail address (like <test@example.com\\>), which browsers will

deal with fine, but Python's XML parser won't.
We already had some code in place to ensure that some entity names were

converted to entity codes, and some other code to convert invalid raw

character codes to a ?. This change combines the two, updating the

entity name conversion to also perform numeric entity value conversion

for any illegal characters.

Unit tests passed.
Verified that this fixed a reported issue on Review Board with these

bad e-mail addresses when diffing fields.
Tested this fix against Python 3 as well on release-2.0.x.