Bring Association back and use it in new Add and Edit data import forms to limit format choices

[dandavies99]

If the wrong format is chosen for a raw data file, it's possible that everything appears to work but no actual measurement data is added.

1. At the frontend, we should make sure that we use the Association to limit Format choices that are relevant to the station that has been selected.
2. At the backend, we should raise a warning (maybe it's some property of DataImportFull that can be made obvious to the user) if no data is imported. Perhaps this should be if the number of columns imported doesn't match the number of columns in the file.

[dalonsoa]

We need to bring Association back into the mix, removed in d5ead03

[dalonsoa]

Ditch in favour of a M2M relationship, better error handling and documentation. See #373 and #333 .