[#873] Show upload option if filestore enabled

* Add a check_filestore helper that checks the config if filestore is
  enabled.
* Check if filestore is enabled and user is logged in before showing
  upload button.
[#873] Use a g.ofs_impl to check filestore status

* Remove the helper function check_datastore
* Create a global varibale called ofs_impl that stores
  the value of the config option
Merge branch '873-upload-button-check'