Allow UI Customization

[flowchartsman]

It would be nice if the user could customize certain aspects of their UI, either via CSS or turning on/off certain swagger-ui features. Of course, the more things that get templatized means the more potentially fragile the generate code to keep the static files up to date becomes, so this will need to be done carefully, and each one will probably need to get hooked into the CI system to surface any potentially-breaking changes from upstream.

• Prebuild some new layouts and themes
• Templatize swagger-initializer.js rather than swapping out the values (plugins/layouts)
• Templatize .css