FR: Filter Highlights and Pins on Document when Student is Selected in the Grading Bar/ SpeedGrader

## Feature Request Form

### When a student's name is selected in the grading bar/SpeedGrader, the Hypothesis sidebar filters the annotations and replies of the selected student. However, the document still shows highlights and pins made by all users.

In the image above, the sidebar is set to show annotations made by the student (Example Student), which shows none, but the document still shows highlights made by other users.

### Your solution
Filter the highlights and pins in the document to show only the ones from the selected student, or the ones the student replied to.

Related ticket: https://app.hubspot.com/contacts/6291320/record/0-5/48577506560
FR Tag: `Filter Highlights/Pins by User`

Start with the grading bar/SpeedGrader student-selection flow and the Hypothesis sidebar's annotation filtering, then trace how document highlights and pins are rendered. The change is complete when the document shows only highlights and pins from the selected student or users they replied to, with behavior covered by the relevant frontend tests.

Written by the indexing model from the issue text.