customization: add ability for helpers to set a different default decline reason

[ghost]

As a helper, I almost always use 'custom' to leave detailed list of things that need fixing. I would like to be able to set that as a default for me.

[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.