[web] cleanup user list modals

Summary:
This diff cleans up the one off styles in the user list modals (friend + block lists) and updates the modal to follow the conventions of the new modal api

Please note that this modal will be fully deprecated in a few weeks in favor for the friend + block list panels in the new web ui. Just threw this diff up so that when I land all the modals, there will be no visual regressions for the time being

Depends on D11067

Test Plan:
Please see the screenshot below

Reviewers: atul, inka

Reviewed By: atul

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D11068