Allow conversations to be sorted by last active

[jhthorsen]

Many chat applications place the conversations with unread messages on top of the list. I would like to have an option to do the same in Convos, so it's easier to see that you have received a new private message.

I'm unsure if this should also apply to channels, or only private messages. Maybe it could apply to channels if "Notify me on new messages" is ticked?

I do believe that the conversations should be grouped as they are today.

[jhthorsen]

fb4076b has a change to filter for conversations with unread messages, but will skip conversations with "60+" unread messages. By accident, you can still find those by searching for "60+" though.

[jhthorsen]

Added 1ce10e1 to filter for channels and private conversations.