Move c3.js to vendor/assets/ and serve separately

The vendor/assets/ directory is a more appropriate place for this file:

http://guides.rubyonrails.org/asset_pipeline.html#asset-organization

> app/assets is for assets that are owned by the application, such as
> custom images, JavaScript files or stylesheets.

> vendor/assets is for assets that are owned by outside entities, such
> as code for JavaScript plugins and CSS frameworks.

Code Climate is currently analysing this file as application code, and
the results are dragging the project GPA down to a hilariously low 0.2:

https://codeclimate.com/github/intercom/pulse/app/assets/javascripts/c3.js

One consequence of moving this file out of app/assets/ is that it isn't
automatically included in application.js any more. Adding it back in is
as easy as `//= require d3`, but I think it's actually better to serve
it separately, since then users won't have to download the same version
of the C3 library again every time the application's JavaScript changes.
Upgrade c3.css to match JS and serve locally

We were using the JavaScript from C3 v0.2.5 with the CSS from v0.1.29.

Mixing and matching these is definitely going to lead to a bad time.
Merge pull request #25 from intercom/elk/vendor_c3

Move C3 library to vendor/assets/ and serve it separately