Base template: rename head block to extra_head

This makes it more clear what's included
JavaScript: add JSHint config

This sets standard JSHint options for the entire project
Templates: refactor JavaScript, CSS into static files

This allows better editor support, linting, etc. and will prepare
for better caching later.

* Created stylesheets and javascript blocks in the base template
* Moved all <script> tags to just before </body> for performance
* Relocated embedded JavaScript and CSS for donation section into
  support/static/{js,css}/
* Relocated embedded <style> to app/static/css/dcpython.css