Add indicators is_latest_version filter

Hmm yes. The overarching issue here is that the concept of a current version only really applies to indicators calculated with the data loaded into the instance. 

The endpoint goal of this task is to have a 'shortcut' filter to get the current version for instance-generated indicators, as well as the indicators for each city imported via the UI. It would seem to me that each city/scenario loaded into the app should have a 'current version' tag.

There are a few ways to go about that, this was one of them. Could additionally filter the indicator-version endpoint to only care about the local city indicators, or we need another way to flag a set of indicators as being valid for a given city/scenario. I'm definitely open to alternative suggestions.