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
```

tests/app/main/test_asset_fingerprinter.py

@@ -0,0 +1,95 @@
+# coding=utf-8
+import os
+
+from unittest import mock
+
+from app.asset_fingerprinter import AssetFingerprinter
+
+
+@mock.patch.object(AssetFingerprinter, 'get_asset_file_contents')
+class TestAssetFingerprint(object):
+    def test_url_format(self, get_file_content_mock):
+        get_file_content_mock.return_value = """
+            body {
+                font-family: nta;
+            }
+        """
+        asset_fingerprinter = AssetFingerprinter(
+            asset_root='/suppliers/static/'
+        )
+        assert (
+            asset_fingerprinter.get_url('application.css') ==
+            '/suppliers/static/application.css?418e6f4a6cdf1142e45c072ed3e1c90a'  # noqa
+        )
+        assert (
+            asset_fingerprinter.get_url('application-ie6.css') ==
+            '/suppliers/static/application-ie6.css?418e6f4a6cdf1142e45c072ed3e1c90a'  # noqa
+        )
+
+    def test_building_file_path(self, get_file_content_mock):
+        get_file_content_mock.return_value = """
+            document.write('Hello world!');
+        """
+        fingerprinter = AssetFingerprinter()
+        fingerprinter.get_url('javascripts/application.js')
+        fingerprinter.get_asset_file_contents.assert_called_with(
+            'app/static/javascripts/application.js'
+        )
+
+    def test_hashes_are_consistent(self, get_file_content_mock):
+        get_file_content_mock.return_value = """
+            body {
+                font-family: nta;
+            }
+        """
+        asset_fingerprinter = AssetFingerprinter()
+        assert (
+            asset_fingerprinter.get_asset_fingerprint('application.css') ==
+            asset_fingerprinter.get_asset_fingerprint('same_contents.css')
+        )
+
+    def test_hashes_are_different_for_different_files(
+        self, get_file_content_mock
+    ):
+        asset_fingerprinter = AssetFingerprinter()
+        get_file_content_mock.return_value = """
+            body {
+                font-family: nta;
+            }
+        """
+        css_hash = asset_fingerprinter.get_asset_fingerprint('application.css')
+        get_file_content_mock.return_value = """
+            document.write('Hello world!');
+        """
+        js_hash = asset_fingerprinter.get_asset_fingerprint('application.js')
+        assert (
+            js_hash != css_hash
+        )
+
+    def test_hash_gets_cached(self, get_file_content_mock):
+        get_file_content_mock.return_value = """
+            body {
+                font-family: nta;
+            }
+        """
+        fingerprinter = AssetFingerprinter()
+        assert (
+            fingerprinter.get_url('application.css') ==
+            '/static/application.css?418e6f4a6cdf1142e45c072ed3e1c90a'
+        )
+        fingerprinter._cache[
+            'application.css'
+        ] = 'a1a1a1'
+        assert (
+            fingerprinter.get_url('application.css') ==
+            'a1a1a1'
+        )
+        fingerprinter.get_asset_file_contents.assert_called_once_with(
+            'app/static/application.css'
+        )
+
+
+class TestAssetFingerprintWithUnicode(object):
+    def test_can_read_self(self):
+        string_with_unicode_character = 'Ralph’s apostrophe'
+        AssetFingerprinter(filesystem_path='tests/app/main/').get_url('test_asset_fingerprinter.py')