Persist spaced repetition

[BenWestgate]

A general solution for spaced repetition training on luks2 encrypted image files placed in $HOME/Persistent (for lack of a better idea now) has been written.

It took me way too long for such a simple feature but it seems like it will pay dividends with #81 where in order to use it a second substantially different passphrase must also be memorized. Without spaced repetition, I didn't feel confident users could quickly memorize two 5 word passphrases to open the Real vs Decoy wallet vaults.

All .img files keep separate intervals based on their success rate, all can be skipped with Esc or 'Cancel' except for Persistent Storage.img since we know they know it and advise them to delete it. Skipping counts as success for the interval adjustment. Starting with the Persistent Storage unlocked is counted as a success for the Persistent Storage interval adjustment.

Intervals are based on due dates, so users will probably be bombarded with training after unlocking their screen or starting Tails after not using it a while until their intervals have grow to be longer than time spent away. Since they on average double each success, this should not be too long.

For example if the interval reaches 30 minutes after Bails is setup, and they follow instructions to test their memory everyday, it will be
60m - Day 2
2h - Day 3
4h - Day 4
8h - Day 5
16h - Day 6
32h -Day 7
Day 8 - no prompt
64h - Day 9
Day 10-12 - no prompt
Day 13 - 128 hours
Day 14-20 - no prompt

Which actually matches our advice: "Everyday at first, then eventually once per week."

and after another success or two we could suggest that they destroy their written copy.

Closes #181

[BenWestgate]

Code review LGTM, untested.

Suggest only testing that the Persistent Storage.img training works correctly, as there won't be any others until #81.

[BenWestgate]

Current issues with this branch:

• Can't skip non-Persistent Storage images.
• Does not shuffle the spaced repetition calls yet.
• Bitcoin Core does not launch if started from an already set up Persistent Storage.
• Have not fully tested from blank installation or the persistence after a reboot.