Update about:tor when Tor Browser is updated

As part of legacy/trac#25694 (moved), I suggested having "Tor Browser has been updated" as a title in about:tor when Tor Browser starts after an update.

The mockup is here https://trac.torproject.org/projects/tor/attachment/ticket/25694/1.3C.jpg

For users interested in view the changelog, we should discuss if an exclusive page for it is the best way to approach it.