Group type icons

[acelaya]

Part of https://github.com/hypothesis/product-backlog/issues/1597

This PR adds icons to groups so that it's easier to identify their type (public, protected and restricted).

[!NOTE]
The new design includes a few other changes which we won't be addressing as part of this PR, like a new way to highlight selected items, moving chevrons to the left side, etc.

Additionally, this PR adds two other improvements:

• Replace "Create private group" text with "Create new group", since users can now create private, protected and public groups (or they will, as soon as the feature is enabled).

• Fix the icon used when one group is uncollapsed. We were not using the right one because it is missing an attribute, but I simply added it inline for now. Before:

  https://github.com/user-attachments/assets/90e8b342-e966-4a19-a176-b2098e00ed57

  After:

  https://github.com/user-attachments/assets/f0fd22a5-8748-4174-bfad-3835ec83b59b

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 99.43%. Comparing base (35a67eb) to head (287c944). Report is 1 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #6667 +/- ## ======================================= Coverage 99.43% 99.43% ======================================= Files 271 271 Lines 10200 10210 +10 Branches 2424 2431 +7 ======================================= + Hits 10142 10152 +10 Misses 58 58 ```

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