Update dashboard API type definitions

[acelaya]

Add FE changes for https://github.com/hypothesis/lms/pull/6341

The changes involve:

• Adjusting API type definitions to match the BE ones.
• Implement the same display_name fallback logic that the BE was previously doing.

In a follow-up PR we can enhance the falling-back logic, and add more information about what that means.

We will also need to improve the ordering so that items where display_name is null, are always in the bottom of the list.

Testing steps

This PR should not introduce any visual changes, so the only possible testing is a sanity check on the dashboard, to verify that's the case.