Add settings menu instead of Color Scheme menu

[dstillman]

Dark Mode doesn't merit this level of prominence. Most people will be happy with Automatic.

And we may want other web-library settings later anyway.

We have "Settings" as a menu option in the account menu, but that of course takes you to the account settings. We could potentially have a Web Library section in there and pass the settings into the web library, but some sort of gear menu within web library would probably be fine.