[web] Introduce onColorSelection prop to ColorSelector component

Summary:
Remove the internal pendingColorSelection state from ColorSelector and replace it with a onColorSelection prop that accepts a callback.

This allows us to reuse the existing possiblyChangedValue and onChangeColor state/functionality in ThreadSettingsModal.

Depends on D3684

Test Plan:
Works as expected:

Reviewers: def-au1t, palys-swm, benschac

Reviewed By: palys-swm, benschac

Subscribers: ashoat, Adrian, karol-bisztyga, yayabosh

Differential Revision: https://phabricator.ashoat.com/D3697