Only allow gocryptfs for new vaults

Nate Grahamrequested to merge
ngraham/plasma-vault:only-allow-gocryptfs-for-new-vaults into master

It's the best maintained and most auditably secure one. Let's not confuse people by giving them a choice of different backends when they create a new vault.

Functionality to choose a backend and use other ones is retained for compatibility with existing vaults. However the backend chooser UI is hidden from the "Create New Vault" wizard.

See also kde-linux/kde-linux#168 (closed)

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