Add images needed for persistent bookmarks test.

They were forgotten, apparently.
Drop TBB's default bridges.
Make the Turbutton button appear in the Tor Browser.

Setting extensions.torbutton.inserted_button to true hides the butten
and is recommended by TBB 4.x's start-tor-browser script when using a
system-wide Tor instance. Probably that's bacause normally an
appropriate environment isn't set up to make e.g. Torbutton's new
identity feature work, so hiding it is a good thing, but in Tails we
do set all that up and want to keep the button.
Hide the Tor Browser's add-on bar.

With Iceweasel we hid it by patching the skeleton profile's
localstore.rdf, which was installed by the package, but now we install
TBB with a live-build hook, and they are run *after* the patches are
applied.
Block disk usage in the Tor Browser via Torbutton.

Setting block_disk to false was another recommendation by TBB 4.x's
start-tor-browser script when using a system-wide Tor instance. Having
disk usage block certainly makes sense in Tails, though, so we set it
to true any way.
Show I2P router console in Tor Browser.

Iceweasel isn't installed any more.
Configure the Tor Browser as the system-wide browser.
Upgrade documentation for the TBB migration.
Allow "remotely" opening new tabs/windows in the Tor Browser.

... so links clicked in other applications can be opened in an already
running Tor Browser instance.
Revive pref lost in TBB migration.
Make Tails browser prefs override TBB's.

I had apparently got it backwards how Firefox deals with the priority
of pref files; it's the lexically first that gets priority, not the
lexically last.
Disable the Tor Browser's automatic update checking.
Enable myspell/hunspell dictionaries in the Tor Browser.
Upgrade to TBB 4.0-alpha-3.
dash doesn't support pipefail.

If unzip fails, the check afterwards will catch the error any way.
Fix typo in comment.
Fix erroneous replace'n'replace in the design doc.