Translate "Loading..." text in inline comments

Summary: Ref T12616. This cements the relationship between ChangesetList (parent container) and Changeset (child) and passes translations down so Changeset can use them to translate the text "Loading..."

Test Plan: Viewed loading changes.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12616

Differential Revision: https://secure.phabricator.com/D17846