[web] Create a modal that allows admins to change a member's role (dropdown menu, actionable buttons)

Summary:
This is the second part of the modal creation, utilizing the earlier Dropdown component and creating the two 'back' and 'save' buttons. Functionality for these buttons will be added up the stack to
make this diff easier to review.

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

Depends on D8137

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

Reviewers: atul, ginsu

Reviewed By: atul

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D8138