[web] Prevent nested scroll in the message results modal

Summary:
We want to prevent nested scrolls inside of the message results modal. This means that for the modal, we want to display the messages in full length (and keep the modal scrollable). However, in the toggle pin modal (the modal that shows the message and allows the user to pin/unpin a message), we want to keep that scrollable since otherwise a long message could overflow off the screen and make the action buttons unclickable.

The easiest solution I thought of for this was to maintain a scrollable prop, set to true if we want to allow the message result to be scrollable if it is long, and false if we want to display the message in full.

Test Plan:
Confirm that the toggle pin modal allows the user to scroll through the message, but the message results modal shows it in full.

Reviewers: michal, inka

Reviewed By: inka

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D8259