Previously, we were passing a binary file to markdown.markdownFromFile in

render_markdown_from_file, but this won't fly in Python 3. It expects

a file opened with an encoding. However, Django does not let you specify

an encoding when opening a file from storage, so that solution is out.

We now just read the entire file into memory, decode it from UTF-8, and

then pass that to markdown.markdown(), which returns text directly.

With this patch applied, text file attachments render correctly

in Review Board on Python 2.7, 3.5, 3.6, and 3.7.