Add signature page that is triggered by mandatory: True

[plocket]

Really we want to figure out all the times where things can actually for realz get messed up by not having a sought var. Just a regular question page will be fine because I don't think it can have proxy vars, but without a sought var on a signature page, I'm not sure we will know which variable to set a signature for.

[plocket]

<input type="hidden" name="_save_as" value="some_base64_var_name"> still holds the var name in this case. That's already where we're getting it from.