make detail view type a trend property

[brownsarahm]

to make the relationship between the app and the fact that trends are extensible more complete

• [x] determine values of the detail view type property needed
• [x] determine if additional properties are needed in app logic or planned adds
• [x] implement detail view type for each vartype mixin (in base_getvars.py)
• [x] change app to use this instead of trend_type