Organize application.js

[mnquintana]

application.js is getting kind of messy.

For greater maintainability in the future, we should turn application.js into strictly a manifest file - it exists only to import our other JS files. We should then compartmentalize each component of our JS into separate partials (ie. all the select2 function calls go in one partial, all the datepicker stuff in another one, etc.)

[mnquintana]

I've found that this isn't as cut-and-dry as partializing our application.css.scss - much of our code is wrapped in a single call to $(document).ready, and I'm reluctant to duplicate that across multiple files. The only things that clearly should be partialized are the functions we define at the top of the file - those can easily be moved into a functions.js.

That said, application.js still needs better organization and clearer comments, so, I'm gonna change the title of this issue to reflect that.

[mnquintana]

We should try this again now that we've changed a lot of the JS.