Add an age gate

[efatsi]

Users have requested the ability to make adult-content stories. Our terms explicitly prohibit some of this, mainly to give us the authority to remove content if it's causing harm. Introducing an optional age gate would be a step towards making that use case more acceptable.

Changes:

• add form elements to Adventure form to toggle age gate on/off, as well as set an age limit
• add splash page (similar to password) if age gate is present that asks "Are you at least __ years old?"