K-9 Mail v5.913

What's Changed

• Don't create notifications when manually refreshing the message list (@cketti)
• Fixed import and export of notification settings (@cketti)
• Fixed bug that could lead to multiple notifications being created for the same message (@cketti)
• Fixed bug that sometimes crashed the app after changing the notification sound (@cketti)
• Added support for messages that specified the character set in multiple ways (@cketti)
• A lot of internal improvements (@cketti)
• Updated translations (many volunteers via Transifex)

Full Changelog: 5.912...5.913