"Unread" is used to mean 2 different things

### Steps to reproduce

1. Using the new room list

3. Click the ... to choose a sort order

### Outcome

#### What did you expect?

I expected the word "Unread" to have a consistent meaning.

#### What happened instead?

* The "Unreads" filter means "filter for only rooms with notifications"
* But Sort -> "Unread first" means "put rooms with unread activity first, even if they have no notifications"

I would LOVE a filter that means "filter for only rooms with unread activity", and I would LOVE it to be called "Unreads", but that is not what we currently have.

#### What do I suggest?

If we really want the "Unreads" filter to keep its current name and meaning, then we need to find a new name for Sort -> "Unread first". Maybe Sort -> "Unread activity first".

If I were in charge, I would:

* Rename "Unreads" filter to "Notifications"
* Keep the name and meaning of Sort -> "Unread first" as it is
* Create a new filter "Unreads" which filters for only rooms with unread activity

Start by locating the new room list UI and the code that defines the Unreads filter and the Unread first sort label. Confirm which naming and filtering behavior the project wants, then update the affected UI and verify that the filter and sort labels have distinct, consistent meanings.

Written by the indexing model from the issue text.