[web] Use tinycolor.equals(a, b) in ColorSelectorButton

Summary:
Replace .toLowercase() string comparison with tinycolor2.equals() which handles that + # prefix existing/missing + 3 char vs 6 char vs 8 char hex etc. for us. Not that we expect to run into those here, but it seems like a more "robust" way to do color comparison?

Depends on D3700 (I guess)

Test Plan:
Things continue to work as expected.

1. Opened ThreadSettingsModal
2. Selected a color
3. Hit save and ensured the thread color was changed to the selected color

Reviewers: palys-swm, def-au1t, benschac

Reviewed By: palys-swm, benschac

Subscribers: ashoat, Adrian, karol-bisztyga, yayabosh

Differential Revision: https://phabricator.ashoat.com/D3701