Removed the option for curators to generate a new CSV export.

As the BioCatalogue takes such a long time to generate the CSV export we have removed the option for curators to do so and instead allow curators only to download a pre-generated one.
There is now a download page that tells them when the last export was generated and how to request a new export be generated if it is not recent enough.
Moved all of the csv export logic from the curation_controller to the curation_helper so that it can be used by a rake task
Stops it from breaking if certain methods return nil
Fixed bug that would crash the generate_stats script. See comment for explanation
Rake task to generate a fresh CSV Export zip.
Fixed registry stats page

As the production stats were so large, using the cache to store the results was becoming problematic. I’ve now got the stats saved in yaml format in the data folder which can be read in by the controller when the page is opened