[web] Pull colors in ColorSelector from selectedThreadColors

Summary:
Slightly better than it was before where we were hardcoding these values twice. Will create the ColorSelectorButton components in a for loop instead of copy/pasting 10 times in subsequent diff.

Depends on D3701

Test Plan:
Continues to work as before, colors show up correctly:

Reviewers: def-au1t, palys-swm

Reviewed By: palys-swm

Subscribers: ashoat, Adrian, karol-bisztyga, benschac, yayabosh

Differential Revision: https://phabricator.ashoat.com/D3702