Improve the semantics and accessibility of the about:tbupdate page

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

• We use paragraphs to separate content.
• We parse and show the release notes as a bullet list.
• We no longer read in the entire ChangeLog.txt file to a javascript string, but only read it line by line since we only need the head of the file.
• We stop using fixed "px" font sizes and use relative sizes instead.
• We change the page layout to give the release notes more horizontal space.
• Add a lang="en-US" attribute to the english text.

Fixes #41393 (closed)