Drag level and scroll

[davidsbailey]

Background

Starts PLAT-430. Previously, #37384 moved the sectionMetrics state up to ActivitiesEditor and made sure to update so that dragging would work when the page was scrolled before dragging starts. This PR deals with a related problem of making sure that dragging will work when scrolling happens after dragging starts.

This PR tackles two specific problems:

1. "drop" target does not follow the cursor (I had to use a mouse with a scrollwheel to capture this behavior)
   

2. dragging doesn't cause the page to scroll
   

Description

Here's what this PR does:

• af741dc "call updateActivitySectionMetrics on scroll while level is being dragged". this recomputes the position (clientY) of the drop targets / ActivitySectionCards within the window, which become stale after scrolling, fixing problem 1.
• 381297b explicitly clear targetActivitySection. This is a minor fix to un-highlight the drop target after the LevelToken is dropped.
• the rest of the commits cause the page to scroll progressively faster when dragging within 100px of the top or bottom of the window (problem 2).

Here is how it looks:

Future work

The following problems are tackled in the next PR:
3. the dragged element does not follow the cursor
4. the logic for reordering levels within the original activity section is broken

Testing story

Manual testing only for drag and drop behavior. Existing test coverage guards against regressions in other features within the lesson editor.

Reviewer Checklist:

• Tests provide adequate coverage
• Privacy and Security impacts have been assessed
• Code is well-commented
• New features are translatable or updates will not break translations
• Relevant documentation has been added or updated
• User impact is well-understood and desirable
• Pull Request is labeled appropriately
• Follow-up work items (including potential tech debt) are tracked and linked

[bethanyaconnor]

Tested locally and looks good. Code makes sense so lgtm!