Refactor importer with tests

refs #4605

- Move as much code as possible out of the DB API
- Split the importer into 2 concepts, code which handles different
  filetypes and code which takes the content of files and imports it into
  Ghost in some way
- Split the import process into 4 steps, load the file into a useful
  format, preprocess the data, do the import, generate a report
- Reporting is currently a no-op
- Adds a basic level of testing