[web] Introduce barebones CommunityCreationModal
Summary:
Absolute minimal CommunityCreationModal that opens when the CommunityCreationButton is clicked and is dismissed when either A. the X is clicked B. the user clicks outside of the Modal.
In subsequent diffs each part of the modal will be added piecewise.
Test Plan:
Looks and behaves as expected:
Reviewers: ashoat, ginsu
Reviewed By: ashoat
Subscribers: tomek
Differential Revision: https://phab.comm.dev/D7721