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.
One thing to note here is that we are skipping tags that may have previously been associated with posts.  I'm not sure if the import will just skip that association or not now.