Fix showing Unicode in the e-mail previews.
Review Request #603 - Created April 25, 2017 and updated
We set custom mimetypes in the e-mail previews for HTML and plain text, but these weren't specifying a character set, causing the browser to fall back on a default and render UTF-8 content incorrectly. These mimetypes have been updated to explicitly specify the UTF-8 character set, fixing rendering. Testing Done: Viewed UTF-8 content in the e-mail previews. Saw that they now render correctly. Reviewed at https://reviews.reviewboard.org/r/8902/