- `RepositoryManager.get_repository` may try to get a repository for which the backend was not loaded
- `username` attribute needed in the `MockPerm` class
- fix rendering of `changeset:` links when there's no reponame specified
- add a few missing `reponame` in changeset resource specifications
Changed:
- links to missing changesets simply have no href attribute
- `export:/path` links should actually refer permanently to the ''latest'' revision of the node, not to the latest at the time the link was created
