[web] Add community creation button to CommunityPicker
Summary:
Just adding this button and hiding it behind isCommunityCreationButtonEnabled flag.
Button doesn't do anything at the moment, in subsequent diffs will introduce modal with the community creation "wizard."
Test Plan:
Looks as expected:
Reviewers: ashoat, ginsu, inka
Reviewed By: ashoat
Subscribers: inka, tomek
Differential Revision: https://phab.comm.dev/D7716