Bug 41524: Read changelogs as UTF-8

We make a number of related changed to improve the semantics and
accessibility of the about:tbupdate page:

+ We parse and show the release notes as a bullet list.
+ We use paragraphs, headings and bullet list to structure content.
+ We stop using fixed "px" font sizes and use relative sizes instead.
+ Add a lang="en-US" and dir="ltr" attribute to the english text.