[web] Add subtitle to Modal

Summary:
As part of the CommunityCreation flow on web, we need to add a subtitle to the CommunityCreationMembersModal (introduced next diff) that says "You may also add members later."

This diff just sets things up for that diff by adding subtitle to Modal component.

Test Plan:
Modals look as expected w/ subtitle:

Modals look as expected w/o subtitle:

Reviewers: ashoat, ginsu

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D7865