import too slow for large corpus files

[nschneid]

django-import-export is not designed for large files. @ablodge is able to get it working locally in dev mode (about 10 min. for STREUSLE usages), but on the server it fails after a minute with a 504 timeout error (and attempts to increase the timeout in apache settings had no effect).

Current workaround: download the database and import everything locally

A better approach in the future might be to test the import locally in the admin interface and then do the real import on the server through the shell: https://gist.github.com/int-ua/b2bcc4bea8227ab14a437900ff424723