Fixed #34235 -- Added ManifestFilesMixin.manifest_hash attribute.

This adds ManifestFilesMixin.manifest_hash attribute exposing a "hash"
of the full manifest. This allows applications to determine when their
static files have changed.

tests/staticfiles_tests/project/documents/staticfiles_v1.json

@@ -0,0 +1,6 @@
+{
+  "version": "1.0",
+  "paths": {
+    "dummy.txt": "dummy.txt"
+  }
+}