Save data to Persistence when it is created (no need to restart)

NOTE: This is one of a series of usability issues that arose from the TAILS OS training workshop on March 19th, 2016 in New York City.

Currently, when creating a persistent volume, one has to restart before using it, as per the documentation: https://tails.boum.org/doc/first_steps/persistence/configure/index.en.html. Unfortunately, when people I’ve trained create a volume, their immediate assumption was that persistence was ready as soon as they set it up. If there is a way to remove the need to restart, that would vastly improve the experience of creating persistent volumes and keep people from becoming frustrated when they create a persistent volume, set up a bunch of things, then restart only to see that they have to go through it all over again.

Feature Branch: feature/11529-enable-persistence-immediately,tps:11529-enable-persistence-immediately