Users should be able to manipulate local files in I2P browser

Originally created by @sajolida on #8280 (Redmine)

Several users asked about the new isolation of I2P. Now that I2P runs in
a dedicated browser, from inside a chroot, I understand that the same
warning than from the Unsafe Browser applies:

« As a consequence, if you download files using the Unsafe Browser it is
not possible to access them outside of the Unsafe Browser itself. »

It was proposed on tails-dev to have a dedicated folder mounted in the chroot for that use.

Related issues

• Related to #5525 (closed)
• Related to #9894 (closed)