Issue #9: remove dependance on libraries module, bundle colorbox in c…

[jenlampton]

Fixes https://github.com/backdrop-contrib/colorbox/issues/9

• bundles colorbox library & removes any reference to libraries project.
• UX cleanup of admin settings form
• Backdrop coding standards cleanup in module
• removed unnecessary files (.make and .variables)
• cleaned up README.md to match required standards

[jenlampton]

Thanks for the review @klonos! :)

[klonos]

Not a very thorough one as I'm 💤 and only checked user-facing text, so could use a second pair of eyes.