Document how to migrate existing KeePassX database

Tails 3.0 will include KeePassX 2.x, while Tails 2.x includes KeePassX 0.4.x. https://www.keepassx.org/news/2015/12/533 lets me think that some manual importation/migration step is needed.

Document that the old database is preserved. That’s good as a backup but once people have done the migration, they can remove it.

Parent Task: #11917 (closed)

Related issues

Related to #10780 (closed)

Original created by @intrigeri on 10956 (Redmine)

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information