[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