Have KeePassX open ~/Persistent/keepassx.kbdx by default

The file extension of KeePassX changed in 3.0 and it should try to open _{/Persistent/keepassx.kbdx by default instead
of}/Persistent/keepassx.kbd.

Feature Branch: bugfix/12260-open-keepassx-database-by-default

Original created by @sajolida on 12260 (Redmine)

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