Remove duplicate posts and tags on import

Issue #806

- Modify sanitize to check for duplicate posts and tags
- Return both data and problems from importData
- Add tests for duplicate posts and tags with fixtures

This only implements the posts and tags part of #806.  Users seemed a little too involved to get done in one hack night.
In the case that there is an association with a dupe tag listed in post_tags, the association will get skipped because the tag with that ID will be missing. So in the case of tags, when a tag is removed, all associations with that tag would need to be updated to point to the ID of the tag that is kept.

For posts, we can simply discard a duplicate. For tags, we need to update the associations.

Users are already effectively de-duplicated based on email address so I think they can be safely ignored.