[web] Use flex-wrap to lay out ColorSelectorButtons in ColorSelector

Summary:
Addresses feedback from @palys-swm and @def-au1t in D3662. It turned out to be way easier than I expected (if I did it right...)

Depends on D3702

Test Plan:
Layout looks correct at different zoom levels:

(the background colors are just to help visualize)

Reviewers: def-au1t, palys-swm

Reviewed By: palys-swm

Subscribers: ashoat, Adrian, karol-bisztyga, benschac, yayabosh, palys-swm, def-au1t

Differential Revision: https://phabricator.ashoat.com/D3703