[web] cleanup members modal

Summary:
This diff cleans up the one off styles in the members modal 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 members list sidebar 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 D11068

Test Plan:
Please see the screenshot below

Reviewers: atul, inka

Reviewed By: atul

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D11069