Use backend-provided page title in repo editor

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/36029

@jschatz1 The tree and blob JSON endpoints now have a Page-Title header that will include the complete page title, the same one that would be displayed if that page were rendered by the backend.

It should be used here: http://gitlab.com/gitlab-org/gitlab-ce/blob/master/app/assets/javascripts/repo/helpers/repo_helper.js#L258