K-9 Mail v6.509

What's Changed

• Cleaned up the message list design (@cketti, @solangevalverde, @Alecaddd)
• Added setting for three different message list densities: compact, default, relaxed (@cketti, @solangevalverde, @Alecaddd)
• Made small changes to the appearance of participant entries in the message details screen (@cketti, @Alecaddd)
• Added validation for email address fields in "Edit identity" screen (@cketti)
• Internal changes (@wmontwe)
• Updated translations (many volunteers)

Full Changelog: 6.508...6.509