Aggregate message-related unreads/quantity information and display them

[PIG208]

This is a followup to:

• Bottom tabs and main menu #1035

In the new design for the main menu, we display unreads next to buttons for pages like inbox, mentions, direct messages, and the number of messages in total for starred messages.

Screenshot

Some pages, like inbox and subscription list (channels page), already do some calculations utilizing the unreads model. Some of that might be useful toward displaying the markers in the main menu. A goal of this issue is to extract a central view-model for unreads to make it easier to reuse the logic, so that we can show them on the main menu.