When modal is open, turn off body scrolling

[jrochkind]

These inline styles are what bootstrap 4 does, discovered through reverse-engineering, although it uses a lot more abstraction to do it. (A ScrollHelper object that can do a lot more things).

This behavior now matches Bootstrap modal behavior, and avoids some weird double scroll within scroll.

Before

Screen.Recording.2024-11-21.at.2.55.47.PM.mov

• Not sure why my test app has nav elements weirdly placed at top, ignore that, unrelated to this PR
• Also note the weird invisible spots on top and scrollbar being out from edge are addressed in Fix scrolling of longer-than-viewport bootstrap modal in a <dialog> #3447
• Note unpleasant weird double visible scrollbars

After

Screen.Recording.2024-11-21.at.2.56.58.PM.mov

• Only one visible scrollbar
• It does not scroll the body behind the modal, it only scrolls enough to see modal
• This matches bootstrap modal behavior

[jrochkind]

Here's how I implemented this locally in a BL8 app, customizing BLJS to do it.

https://github.com/sciencehistory/scihist_digicoll/blob/659748a2544aa7bd2ef4b1c807ee456395bfcbbb/app/frontend/javascript/blacklight_setup.js#L32-L49