Show notifications chronologically, make flags a type of notification, and allow filtering notifications by type

## Chronological order

Currently notifications are grouped by discussion, and thus they are not displayed entirely chronologically. The discussion with the most recent notification is shown first, but much older notifications may be included in that, and then the second-most recent may be pushed down. For example, currently the first discussion in my notifications list has three notifications under it: one from 2 days ago, then two from 10 days ago. Only then is the next most recent notification (3 days ago) from a different discussion shown.

I think this is a bit counter-intuitive, seeing as notifications are all about reporting what's happened most recently. In my experience, there is little tangible benefit to having them grouped this way, apart from saving a bit of room by not repeating discussion titles. So I think pure chronological order would be better than grouping by discussion.
## Flags and filtering

Also, flags are in their own separate dropdown, independent of notifications. This is OK, but it's not ideal for the tab bar in the new mobile UI (#867). While there would be room for another tab for flags, I don't think they are important enough to be front-and-center like that. I think it makes more sense, and will simplify implementation, to merge them into notifications as a new notification "type".

In order to still be able to view a list of flags, we should add a dropdown to filter by notification type. Flag notifications would still disappear when the flag is dealt with.

Here's a mockup implementing the above suggestions. I've also added color to the notification icons (making them like badges), to make them more visually distinctive from each other.


Thoughts?

No files, tests, or entry points are identified in the issue. Start by locating the notifications and flags UI, then compare the implementation with the linked mockup and the mobile tab-bar requirements in #867. Done means notifications are chronological, flags appear as a filterable notification type, and resolved flags disappear.

Written by the indexing model from the issue text.