refactor: Move by_conversation_and_time_uri to hash_util.

This removes the 100% coverage on hash_util, but we are
pretty careful about not messing with this code.

tools/test-js-with-node

@@ -43,7 +43,6 @@ enforce_fully_covered = {
     'static/js/fenced_code.js',
     'static/js/fetch_status.js',
     'static/js/filter.js',
-    'static/js/hash_util.js',
     'static/js/keydown_util.js',
     'static/js/markdown.js',
     'static/js/message_store.js',