Fix bugs that stop import of standard plugin CSV export

Making changes that better reflect the default export file of the plugin. Specifically:
- Now only gives a 'bad number of columns' error if number of columns is *less than* 16 (the default export amount of columns)
- 'is_confirmed' binary if statement changed to reflect the actual values used in the export file (was showing items as confirmed if the column had any contents)
- Add '@' to allowed characters list, as it was being stripped from the email field on imports, effectively breaking them

Ideally the last change would be done differently, but this works for now