Change default protocol (IMAP vs POP3) depending on whether persistence is enabled or not
_Originally created by @kytv on [#10574 (Redmine)](https://public-redmine-archive.tails.boum.org/code/issues/10574)_ In tails/tails#9303 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: tails/tails#5663
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