0.15.1: Read and unread that make sense

Two small fixes that make everyday mail feel right:

• Read / unread on conversations. A conversation with an unread email inside now counts as unread, so its button offers Mark as read, and that reads the whole conversation at once, like in Gmail. Selecting several conversations and choosing Mark as read does the same.
• Reply type on Roundcube 1.7. Switching between Reply, Reply all and Forward in the inline reply works again before you start writing.