Backup MetaMask state to a file, and restore state from file

We should allow backing up the MetaMask state to a file, and restoring from this file. This would let users create file backups, to ensure they can restore their encrypted vault and other wallet data (account names, address book, etc.) in the event their primary wallet instance is lost or corrupted in some way.

We have already implemented a backup solution using 3Box (See #1459 and #6876), but at the moment it's still experimental, and doesn't backup the entire state. Many users might prefer to use a file backup anyway, to control manually when backups are taken and how they are stored.

We should probably not encourage most users to use this feature, as it could very easily be accidentally misused and expose users to security/privacy risks if they aren't careful with how this file is stored. We should include a warning/disclaimer of some sort alongside this option, to ensure users are adequately informed of the risks.

Related: #2340, #5913

The issue does not name files, tests, or entry points. First define the backup format and restoration scope, including the encrypted vault, account names, address book, and the security warning; done means the full state can be backed up and restored without exposing users to avoidable storage risks.

Written by the indexing model from the issue text.