refactor: Remove redundancy & improve consistency of UnreadCounts.

[neiljp]

This PR is a follow-up to some renaming done for the Index data, now for UnreadCounts.

This improves how clearly and concisely the code reads by removing the redundant unread_ prefix from 3 entries, while also making them more consistent with the other entries (eg _pms suffix).

While amending the tests it looks clear we would benefit from making more generalizations from solo to include group pms in more cases, both in tests and potentially in code.

Tests amended.

[zulipbot]

Heads up @neiljp, we just merged some commits that conflict with the changes your made in this pull request! You can review this repository's recent commits to see where the conflicts occur. Please rebase your feature branch against the upstream/master branch and resolve your pull request's merge conflicts accordingly.