SW-261: Return to datatable and change CSV or columns

[wheelsandcogs]

Users can now return to the datatable summary:

And then choose to either change the upload or the column source assignment:

Other changes:

Dataset is fetched upfront by middleware
There's a new middleware fetchDataset that calls the api to get a dataset with all currently available relations hydrated before each route executes. This simplifies having to worry about what to fetch in most cases. If we find that the response starts becoming too big or slow, we can look at making some of the relations optional, but currently it's like 20-30ms so I'm not expecting this to be an issue.

Most GET/POST routes are now handled by the same controller
e.g.

publish.get('/title', provideTitle);
publish.post('/title', upload.none(), provideTitle);

We generally need the same or similar things for displaying the original page and errors after a form submit, so this reduces the amount of code duplication a fair bit.