thread: Add searchable view to ThreadBody

This will be useful for plugins which might need to scan the message body
regardless of its encoding. This will also be required for full-text indexes
which will not need to index HTML tags, for instance.
mail: No auto responses for auto-reply mails

This requires extra processing in the mail backend, perhaps through a
plugin, which would detect and flag a message as an auto-reply. Auto-replies
are added to the ticket thread, but have the automated responses disabled.