Added an automatic migration system for notes before the #1 redesign.
Ths system works as follows:
1. Checks the password system used.
2. If it's the old system, the migration is started.
3. The data directory is backed up.
4. The notes are decrypted using the old password.
5. The notes are encrypted and saved using the `KDF(password)`
This commit also adds documentation related to future migrations of the "password system" and which migrations will be supported by each future version.
This documents also showcases that when v1 is released, support for `PasswordSystem::V0` will be completely removed.
