consider map scaling

[mmontesanonyc]

See Restaurants with A grades. Currently, all maps are 'internally scaled' - with mins and maxes that reflect that year's min and max.

RwAG has a clear trend over time of improvement. Scaling the min and max across years would let a time browse of the map communicate this more clearly.

Ideas:

• Allow uploading user to set mins and maxes (drawbacks: requires manual intervention
• Create additional metadata flag in indicators.json that tells the code to get mins and maxes for the measure across all years - and applies that
• Allow uploading user to set the min and max from a specified year
• Or - create another view that's designed to communicate change over time.

Preference is for an approach that doesn't require a lot of manual control over parameters, so that we don't create an application that has too much refined customization or can create a lot of messy edge cases.

[mmontesanonyc]

Adding to #240 and closing till we decide to tackle.