customization: add ability for helpers to set a custom user notification message

Currently helpers can tick the 'notify user' box and the user would be notified that their submission failed, using a template. I would like to see helpers able to customize the message content, so they can write their own text instead of the template.

[theopolisme]

Thanks for the suggestion. We've got a preferences system in the works (actually, it's basically done -- I just need to merge the branch) and this should be implemented with #83.