[web] Create a modal that allows admins to change a member's role (modal, description, avatar)

Summary:
This is 1/2 of the modal that allows admins to modify member roles. This will just cover the actual modal, the description, and displaying the user avatar and info.

https://linear.app/comm/issue/ENG-4038/create-a-modal-that-allows-admins-to-change-a-members-role

Depends on D8136

Test Plan:
Please see below a screenshot of what the modal looks like for now

Reviewers: atul, ginsu

Reviewed By: ginsu

Subscribers: ashoat, tomek, ted

Differential Revision: https://phab.comm.dev/D8137