Feature/refactor versions

The previous 'is_latest_version' and 'version' scheme is replaced with the idea that IndicatorJobs have IDs and indicators are associated with identifiable calculation jobs.

'is_latest' is equivalent to the highest job id that has a 'complete' status.
Should this filter on city_name?

Since indicatorjobs will be submitted when the user imports a csv for example, it could be the case that this returns a job that isn't for the currently loaded city?