Add an about:tbupdate page that displays the first section from
TorBrowser/Docs/ChangeLog.txt and includes a link to the remote
post-update page (typically our blog entry for the release).

Always load about:tbupdate in a content process, but implement the
code that reads the file system (changelog) in the chrome process
for compatibility with future sandboxing efforts.

Also fix bug 29440. Now about:tbupdate is styled as a fairly simple
changelog page that is designed to be displayed via a link that is on
about:tor.