Fix premature (and incomplete) HTML-escaping of links

[mentalisttraceur]

This fixes three bugs:

1. links were being prematurely HTML-escaped (when being parsed into AST instead of waiting until rendering to HTML) ( Certain characters in markdown code incorrectly parsed (e.g., &) #394 )
2. image directive was double-escaping target links, and
3. image directive wasn't escaping src links at all.

Same idea as the fix in #393 - move the HTML-escaping out of the parse and into the rendering.

Tests updated to catch all three bugs.

You can merge this and #393 in any order (no merge conflicts either way).

[veenstrajelmer]

Thanks for this PR. I locally checked out the branch to check its effect in my sphinx-build usecase. I just wanted to confirm that your PR indeed fixes the issue I encountered.

[veenstrajelmer]

And additionally, #393 fixes another issue I encountered only recently for the first time. Thanks for these very helpful contributions!