[web] Introduce ColorSelectorButton component with basic styling

Summary:
Introduce ColorSelectorButton component with basic styling.

Note: The actual functionality will be introduced in subsequent diffs.

Test Plan:
Looks as expected (each of the colored circles is a ColorSelectorButton):

Reviewers: palys-swm, def-au1t, benschac

Reviewed By: def-au1t

Subscribers: Adrian, karol-bisztyga, benschac, yayabosh

Differential Revision: https://phabricator.ashoat.com/D3660