Add Permissions Button color customization

[bjtitus]

Adds permissions button colors properties to KanvasColors:

• KanvasColors.permissionsButtonColor
• KanvasColors.permissionsButtonAcceptedBackgroundColor

We want to customize these colors a bit for WordPress iOS: #54

[danielebogo]

LGTM! One question:
Do you think the function func cameraColors() -> KanvasColors in KanvasCustomUI may return .shared instead? It seems like the KanvasColors value returned there is the same generated by the shared function.

[bjtitus]

Thanks @danielebogo! 😀

It looks like a few of the colors are slightly different, causing 42 tests to fail the snapshot diffs if I switch to KanvasColors.shared. I guess it's kind of nice that we are testing that the overridden colors work 🤷‍♂️.

I think the Colors + Fonts override API should change so that we aren't mutating the static KanvasColors.shared property. I feel like that's not a very safe way to do things, but it would have required more extensive code changes to access them inside of Kanvas.

[danielebogo]

Thanks @bjtitus !