Debug mode support for assets

[actual-saurabh]

Summary of Changes:

1. Extend support for SCRIPT_DEBUG to all enqueued assets, whenever available, including for
   1. bundled third-party assets
   2. assets loaded by CDN
2. Optimise enqueuing mechanism by replacing multiple function calls and variables with single global constants.

[thomasplevy]

@actual-saurabh Why'd you close this?

[actual-saurabh]

Oh, sorry. I was cleaning up branches on my fork and deleted this one too. Restored the branch. Should I pull from master, merge and update the pull request?

[thomasplevy]

please! I hadn't merged it in yet but it looks solid and I'd like to get it into core

[thomasplevy]

@actual-saurabh there's an issue with this... most of the JS files are only added to the assets directory as minified versions. The gulp tasks that concat & minify JS need to create a concatenated only non minified version in order for this to fully function, otherwise with script debug enabled most of the scripts will 404

[actual-saurabh]

I have added a new task to minify vendor scripts to maintain consistency. See: https://github.com/actual-saurabh/lifterlms/tree/feature-debug-enqueue/assets/js/vendor

I then added this as a dependency in the original task. I also modified the original process-scripts task to also copy over unminified assets: https://github.com/actual-saurabh/lifterlms/tree/feature-debug-enqueue/assets/js

[actual-saurabh]

@thomasplevy #504, #505 describe what look like legacy remnants in assets that don't seem to be used anymore. Can I go ahead and delete them?

Apart from these two, #503 describes two other scripts that have minified version in the distribution but no unminified sources to build from.