Renamed content blocks for expanded use cases.

I've found that there are several scenarios where the content block `:extra_js` name does not make much sense. I think we'd be better served with `:head` and `:body_end` content blocks. Here are some scenario examples:

- [Including page-specific modal markup on top level body so that it works in Firefox](https://github.com/centresource/athlete-endeavors/blob/8a4e3be394dbee40ab50c68c86d6a99809617a5e/app/views/athletes/index.html.erb#L50-L52)
- [Including JavaScript/stylesheets in the head and JavaScript in the body for MapBox to initialize properly](https://github.com/centresource/cs-website/blob/master/app/views/site/contact.html.erb#L1-L14)