hark: Notifications Improvements/Streamlining

[urcades]

Taking into account some feedback from #1204 and #1201, I'm providing a few updates to the notifications controls that should 1) lessen the amount of conceptual overhead and 2) further clarify what each button does, and what notification states mean.

Here's the current state:

While a few improvements have been made, such as unifying the "Mark as Read" and "Archive" labels/functions into a singular "Archive", there remain a few things we can do to clarify the interface.

Some lingering issues:

1. Controls remain clustered to the top left corner of the interface, take some time to conceptually tease apart
2. Surfacing "Preferences" in an interface that will frequently be used and expanding the surface area of "Preferences" needlessly
3. "Seen" and "Unseen" labels add noise and "yet another thing to understand" needlessly — This designation should be completely eliminated

Each of these three lingering issue are addressed in this rearrangement of the primary notifications interface:

Note the following:

1. There is now a separation between the "notifications"/"archive" split navigation toggle and the "archive all" button, ensuring the interface is more readable at a glance
2. "Notifications Preferences" button is removed — one less thing to think about
3. "Seen" and "Unseen" labels are removed — another two less things to think about

There remain a few resulting mechanics here, which will be explained in the comment after this post.

[urcades]

With the removal of labels that signify "seen/unseen" in the inbox, it's worth rethinking exactly what the notifications badge is representing:

Currently the badge's count AND color state map to how many notifications are "unseen" by a user, which is additionally designated by the number of "highlighted" items in the notifications list:

https://user-images.githubusercontent.com/1195363/134830444-2475f4b7-01de-441d-82d3-a1df74cab95a.mov

I'd argue that in addition to removing the "seen" and "unseen" labels in the name of "less moving parts" in the notifications interface, we could stand to simplify the designation of whether notifications are "seen" (a boolean) by only relying on the color of the badge (a boolean) — The count in the badge could then be relegated to tracking how many notifications in the inbox have yet to be interacted with, whether they are clicked-through or archived.

Both the new badge count and new badge color behavior are represented in the following prototype:

Prototype A demonstrates how clicking into the notifications interface doesn't decrement the badge count, but shifts its color to a diminutive gray:

https://user-images.githubusercontent.com/1195363/134833617-3efa84b9-0b43-47ca-b3c7-94ce8fce8a23.mov

Prototype B demonstrates that clicking into the notifications interface and archiving a notification decrements the badge count (and obviously sets the badge to a diminutive gray):

https://user-images.githubusercontent.com/1195363/134831920-42c2d44c-268c-43f0-9bb9-2206be38178e.mov