[web] Introduce ColorSelector component with basic styling

Summary:
Introduce ColorSelector component with basic layout/styling.

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

Depends on D3660

Test Plan:
Looks as expected:

Reviewers: palys-swm, def-au1t, benschac

Reviewed By: palys-swm

Subscribers: Adrian, karol-bisztyga, benschac, yayabosh

Differential Revision: https://phabricator.ashoat.com/D3662