Explain why it is complicated to propose Tails devices for sale

Originally created by @sajolida on #7269 (Redmine)

If burn on DVD, then the DVD gets outdated after 6 weeks at most.

If installed onto a USB stick, then:

• People would not be able to verify that the Tails on the USB stick
  is genuine. Trusting a certain Tails is a genuine one is either based
  on cryptographic verification, or on personal trust (someone you
  trust cloning a Tails for you). But one Tails is installed on a USB
  stick it is not possible to use our recommended verification
  techniques anymore. Being able of trusting the Tails you use is
  something we really care about, so customers would have to be warned
  out that.

• We release a new version every 6 weeks. Given that schedule, it
  would not be possible to get Tails preinstalled from factory and the
  stock of USB stick would have to be upgraded every 6 weeks.

• Possibly even worse, distributing cloned Tails USB sticks, would sabotage the plausible deniability feature we want to add (#5929) regarding the persistent volume. See https://lists.torproject.org/pipermail/tor-talk/2013-November/031095.html for details.

Related issues

• Related to #5929
• Related to #7270 (closed)
• Related to #7496