Get the list of persistence presets from the Tails being tested.

... instead of maintaining a static list that often gets out-dated,
and makes it impossible to run usb_install.feature with an --old-iso
that has a different set of presets. That's still not possible, but
this is one step in that direction.
Actually test permissions of link-type persistent directories.

... such as the one for the "dot files" persistence preset. The
removed code was supposed to do that, but was bugged (we didn't loop
over the `all_persistent_dirs` we crafted for that purpose).
Improve variable name.
Make it possible to use an old iso with different persistence presets.

... by only checking that the persistence presets that were available
when installing Tails from the old iso are enabled. Any new presets in
the "current" iso are not checked.
We should not ignore missing persistent source directories.

That should imply that something has gone very wrong. I'm not sure why
we've been ignoring them (certainly it would deserve a comment!) but I
can imagine there was some thought about the "old iso with fewer
presets than the current iso" situation that was just solved, but that
wouldn't be enough for some of the other checks.
Check access rights of all persistent source directories.

We don't want the persistent system directories to be too restrictive
nor to lax (at least the current ones).
Improve some steps' names grammatically and descriptively.
Slightly simplify code, and remove magic number.

This is "slightly" more straigh-forward, and not having to bump the
number is nice, especially now when the list of presets is dynamically
fetched from the Tails system being tested. From now on there should
be no need to add support for new persistence presets in the test
suite without breaking it, and they will benefit from the very basic
and general testing we do of each of them.
Verify that all available presets were enabled.

Mostly to verify that the crazy keyboard commands used to check them
in t-p-s works.
Translate the v1.2 release note in french
Install all localized Iceweasel search plugins (Closes: #8139).

That is, except a few that we explicitly filter out.
More pointers to Freepto WIP.
Merge branch 'stable' into feature/7512-Make-GnuPG-config-closer-to-duraconf-reworked
Add a blueprint for #7631.
Formatting.
More meaningful numbers.
Unifiying titles.
Fix formatting.
Fix grammar.
Make tor-browser desktop file handle URL parameters.