Disable the preview (experimental) button for resources that won't work

[nfee006]

5. PVI [Standard web]
   For individual datasets, the button « Preview (experimental) » does not seem to work at all. NF note: this might be for resources that point to folders. Need to implement a way of turning this button off for certain resources. This is not conformity so I will deal with it.
6. PVI [Standard web]
   For a dataset, if you download a txt (ex. https://datatest.asc-csa.gc.ca/fr/dataset/02969436-8c0b-4e6e-ad40-781cdb43cf24), it does not work. With firefox, it’s possible to select notepad but it gives an error related to the file or extension (the other browsers just open a new tab). NF note: this is because it’s actually pointing to a folder and not an individual file. This will be fixed if I implement the solution for 5.

Temporary measure: changing the file format so that CKAN doesn't think that it can preview it.

@coleweusthem FYI