Bugfix 1019925 - Images with Data-URI's now show

I think it's an awesome improvement to have a dedicated test file and test case for the sanitization in the back-end, thank you!  It's especially great that this is a standalone unit test; as you may have noticed, we get a lot of coverage from what are essentially end-to-end tests, which works and is desirable, but was us cutting corners.

To help clarify this is a unit test, I think it would be handy to suffix _unit on the end, but I leave the call up to you as to whether you think that would help people curse less when coming up to speed on the email app :).  If you do change, be sure to update test-files.json too.

I think it would also be beneficial to reference that there is some coverage in test_mail_html.js in the comment.  Also, if you might add a very minimal data URI to the kitchen-sink case at https://github.com/mozilla-b2g/gaia-email-libs-and-more/blob/master/test/unit/test_mail_html.js#L19 that tests it, that would be handy too.  The goal there is that since that code path passes things through the entire body-part parsing stage, we can be sure that the logic is really running.