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/asset_fingerprinter.py

@@ -0,0 +1,46 @@
+import hashlib
+import codecs
+
+
+class AssetFingerprinter(object):
+    """
+        Get a unique hash for an asset file, so that it doesn't stay cached
+        when it changes
+
+        Usage:
+
+            in the application
+            template_data.asset_fingerprinter = AssetFingerprinter()
+
+            where template data is how you pass variables to every template.
+
+            in template.html:
+            {{ asset_fingerprinter.get_url('stylesheets/application.css') }}
+
+        * 'app/static' is assumed to be the root for all asset files
+    """
+
+    def __init__(self, asset_root='/static/', filesystem_path='app/static/'):
+        self._cache = {}
+        self._asset_root = asset_root
+        self._filesystem_path = filesystem_path
+
+    def get_url(self, asset_path):
+        if asset_path not in self._cache:
+            self._cache[asset_path] = (
+                self._asset_root +
+                asset_path +
+                '?' +
+                self.get_asset_fingerprint(self._filesystem_path + asset_path)
+            )
+        return self._cache[asset_path]
+
+    def get_asset_fingerprint(self, asset_file_path):
+        return hashlib.md5(
+            self.get_asset_file_contents(asset_file_path).encode('utf-8')
+        ).hexdigest()
+
+    def get_asset_file_contents(self, asset_file_path):
+        with codecs.open(asset_file_path, encoding='utf-8') as asset_file:
+            contents = asset_file.read()
+        return contents