https://www.pivotaltracker.com/story/show/113448149
This commit adds a query string to assets URLs which is generated from a hash
of the file contents. When asset files are changed they will now be served from
a different URL, which means they wont be loaded from browser cache.
This is similar to how GOV.UK template adds its version number as a querystring
parameter for its assets.
This is mostly copied from Digital Marketplace utils:
https://github.com/alphagov/digitalmarketplace-utils/pull/102
They have it in a shared codebase, we only have one frontend app so don’t need
to do that.
Usage in a template:
``` jinja
{{ asset_fingerprinter.get_url('stylesheets/application.css') }}
```
Output:
```
static/stylesheets/application.css?418e6f4a6cdf1142e45c072ed3e1c90a
```
