Make it easier for developers to customize questions about users list - one question for user 0/i and adaptive use of "you"

[nonprofittechy]

This:

• Removes the special case questions that refer to users[0], simplifying the ability to override them. (fix Remove the references to [0] and just use one question people can override for [i] #851)
• Adds a new variable, al_person_answering, that defaults to "user", to make the text responsive in a way that addresses the special case for user 0 without assuming that the form is being completed by the user. (Anywhere that we say "you" we should consider how to account for forms that are always used by an attorney #109)

Authors have the freedom to add their own question defining al_person_answering, but this is hardcoded in a code block in the stock version of Assembly Line to avoid disrupting the behavior of existing interviews. The only specially handled value is "user". Authors are free to specify options like "attorney", "helper", etc.

Fix #109, fix #851