[CMI] Color module needs to regenerate CSS files during config import

[quicksketch]

I ran across this while fixing #752. We need to have Color module regenerate CSS files when doing a full site config import.

To reproduce:

• Change Bartik settings to a particular color scheme.
• Export config.
• Change Bartik settings again to a different color scheme.
• Import the config from step 2.
• Site is now missing CSS/colors in Bartik theme.

[quicksketch]

PR filed at backdrop/backdrop#760, but it needs tests for this to be merged.

[hosef]

There were a lot of conflicts with the original pull request because of some upstream changes so I copied the changes into a new branch and created a new pull request for this issue.
I also added a new test to the Color module, and made the tests that were there comply with Drupal coding standards.

[herbdool]

@hosef tested with @quicksketch's steps above and it works for me. And looks like the new tests pass. Great work!

[quicksketch]

I've merged backdrop/backdrop#2222 into 1.x for 1.11.0. Thanks @hosef! That test implementation is simpler than I had expected, nice work!

We have a merge conflict that prevents cherry-picking back into 1.10.x. If it's acceptable that this only applies to 1.11.0, then we can leave as-is. If we need a backport, could you reroll the PR against 1.10.x?

Closing for now, please reopen if we make a backport.