[web] fix button in add friends modal not visible on small screens

Summary:
PLEASE NOTE THAT THIS DIFF AND SUBSEQUENT DIFFS IN THIS STACK WILL NOT BE LANDED UNTIL MORE OF THE REDESIGN IS READY SINCE THIS WILL CAUSE REGRESSIONS IN PROD

The cta button in the add friend/block friend modal were not visible on smaller screens. This is because we were setting a fixed height of that would extend some screens. This diff fixes that bug + some copy that was requested in the linear task.

Linear task: https://linear.app/comm/issue/ENG-6387/buttons-in-add-friends-modal-not-visible-on-small-screens

Depends on D10823

Test Plan:
Please see the screenshots below

before:

after:

Reviewers: atul, inka, ashoat

Reviewed By: atul

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D10849