Vacation handling should be case insensitive

In most cases James treats mail accounts as case insensitive. A notable exception is vacation handling. So while a mail gets delivered to the same Inbox regardless of recipient address case,  the sender won't get a vacation notification unless the recipient address case matches exactly.

I propose we consolidate the behavior and make vacation handling case insensitive as well, to better fit user expectations.

Note that this change should only affect our own accounts. We cannot know if other providers treat their mail addresses as case sensitive or not. So if they send multiple mails with the sender address in different case, we should still send multiple vacation notifications to be on the safe side.