[web] Refactor the members modal to support any variety of role labels

Summary: In preparation to support multiple custom roles, we want to remove any hard coding of 'Admins' and 'Members' in the members modal, and instead display them based on the member's role ID. We associate the ID to the role name available in the threadInfo. Note - since the "parent admin" doesn't have a role ID associated to it and is 'approximated' by it's permissions, I check for that first still.

Test Plan:
Please see the screenshots indicating how it will look for custom roles (based on the Figma, we don't display 'Members', so that's why they're excluded in the code, but for testing I show them in the SS)

Reviewers: atul, ginsu

Reviewed By: atul, ginsu

Subscribers: ted, ashoat, tomek

Differential Revision: https://phab.comm.dev/D8067