Clarify default web pages to say that unencrypted messages cannot be received

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See forum topic: https://support.delta.chat/t/public-chatmail-servers-prohibits-sending-out-un-encrypted-messages-but-they-also-prohibit-receiving-unencrypted-messages-and-nothing-mentions-that/4236

We cannot just say that receiving unencrypted messages is not allowed, because it is only true for new users.