[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