Expose MajorityLocation

[jc-harrison]

Expose MajorityLocation through the API, and add a corresponding flowclient query spec. This should be added to ReferenceLocationSchema and InputToFlowsSchema (while we're here, I'm wondering whether there's any reason not to add ReferenceLocationSchema to InputToFlowsSchema, to avoid duplication here).

The 'subscriber_location_weights' parameter should accept any appropriate exposed query (i.e. those with subscriber, location and numeric 'value' columns) - for the moment, I think this will only be LocationVisits.

I think 'weight_column' should be hard-coded to 'value', which will require renaming the 'dl_count' column of LocationVisits.

[jc-harrison]

While we're addressing this, we should move MajorityLocation from 'features/location' to 'features/subscriber' - I failed to catch that one in review.