Change default protocol (IMAP vs POP3) depending on whether persistence is enabled or not

In #9303 (closed) we decided that we should have POP as the default protocol if persistence is enabled and IMAP as the default if persistence is not enabled.

This ticket is for the actual implementation.

Feature Branch: feature/6154-secure-autoconfig-in-icedove

Parent Task: #5663 (closed)

Original created by @kytv on 10574 (Redmine)

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