Make URLs for assets cache-proof

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
```

app/__init__.py

@@ -18,6 +18,7 @@ from app.notify_client.user_api_client import UserApiClient
 from app.notify_client.job_api_client import JobApiClient
 from app.notify_client.status_api_client import StatusApiClient
 from app.its_dangerous_session import ItsdangerousSessionInterface
+from app.asset_fingerprinter import AssetFingerprinter
 import app.proxy_fix
 from config import configs
 from utils import logging
@@ -30,6 +31,7 @@ user_api_client = UserApiClient()
 api_key_api_client = ApiKeyApiClient()
 job_api_client = JobApiClient()
 status_api_client = StatusApiClient()
+asset_fingerprinter = AssetFingerprinter()
 
 
 def create_app(config_name, config_overrides=None):
@@ -106,7 +108,8 @@ def init_app(app, config_overrides):
     def inject_global_template_variables():
         return {
             'asset_path': '/static/',
-            'header_colour': app.config['HEADER_COLOUR']
+            'header_colour': app.config['HEADER_COLOUR'],
+            'asset_url': asset_fingerprinter.get_url
         }