[web] Fix modal styles

Summary:
issue: ENG-8679
The issue was that the pinned messages modal div was not scrollable, but fixing this revealed another issue: contents of Modal were wrapped by modalContentContainer which allowed its content to extend it.

Test Plan:
Checked ALL places where Modal component is used. Checked that after these changes they all work correctly. Here are modals that got changed:

Here is before and after for the search component (using prod and dev):

Reviewers: tomek, ashoat

Reviewed By: tomek

Differential Revision: https://phab.comm.dev/D13191