Implement Scroll to Bottom on Timeline V2 (modal branch)

[marcustyphoon]

likely to be rebased.

also I can't PR a branch against an external PR branch; that's sad

Description

This implements Scroll to Bottom on V2 timelines.

As noted in the linked issue, on V2 timelines, the loader element we need to detect so that we keep scrolling conditionally:

• Isn't the last child of its parent anymore (and doesn't have a classname iirc?)
• Doesn't exist unless the timeline is actively loading content

Thus, this:

• Fixes the selector
• Implements new logic: scrolling will stop after a short delay unless a loader is added during the delay window. (This is basically equivalent to "debounce the stopscrolling function" in some meaningful sense, though I believe literally doing that has some bad edge cases. Didn't think this through much; if that works we should do it so the code is easier to understand.)

Resolves #1491

Testing steps