Grant permission to create content on behalf of another user

There are a couple of situations where it would be useful to be able to create content on behalf of another user:
1. Administrators often need to be able to post content on behalf of their manager
2. When bringing old content into Confluence it would be ideal to be able to maintain the original author without requiring that author to do the editing themselves

This relates to CONF-1711 which requires similar functionality for managing post dates. I guess what is really needed is a general metadata update privilege. Clearly this should not be a globally available privilege as it can be abused. I'd propose allowing administrators to be able to do this, and they should also to be able to grant permissions to others. It probably also needs to be a per-space setting.