[web] Basic CommunityCreationModal layout/styling

Summary:
Introduce some of the layout/styling for the CommunityCreationModal.

This diff is mostly about placing the necessary elements, subsequent diffs will handle enabling functionality.

Test Plan:
Looks as expected (w/ avatar placeholder):

Looks as expected (w/o avatar placeholder):

Reviewers: ashoat, ginsu

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D7764