v3 ideas

[MartijnR]

• remove theme parameter from all endpoints?
• add POST /survey/theme to change theme centrally instead
• or instead of previous 2 points, let theme override only be valid for current session
• remove instance_id parameter (and instead extract this from submitted instance - slower but a simpler API)
• let all requests return a url property
• perhaps remove the /survey/all endpoint (which is the reason that each return property has a different name)
• maybe use /survey/collect instead of /survey and /instance/edit instead of /instance? - Not sure
• maybe "single" should be modifier such as iframe?