Add support for "Recovery Phrases" #674
Description
The Tahoe-LAFS Software Foundation has recently pledged some funding to "research/implement 'recovery phrases' for rootcap management". (Some earlier, experimental work towards recovery phrases was started some time ago in this repo but such work was never finalized.)
Gridsync should strive to support the forthcoming "Recovery Phrases" feature in whatever final form it may take (for example, by updating the setup UI/UX to allow users to create and restore from Recovery Phrases) -- in addition to supporting the current "Recovery Keys" as they exist presently.