Allow conversations to be sorted by last active

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.

No files, tests, or entry points are named. First clarify whether last-active sorting applies to private messages, channels, or notification-enabled channels, then inspect the existing conversation grouping; done means the new option preserves grouping while ordering conversations as specified.

Written by the indexing model from the issue text.