Migration: location based rule tests

[rowanseymour]

HasWard
HasDistrict
HasState

We probably don't want to send all adminboundary information with every request, so need a solution that either caches this, calls back to the caller, or accesses an external service.

[nicpottier]

Assets with lazy loading?

[rowanseymour]

I think we agree now this should be handled via a generic asset caching system, so the question is what kind of asset is it. Doesn't make much sense to model each location as a separate asset, so I think it's something that describes all the admin boundaries for a country together, e.g.

"assets": [
  {
    "type": "boundary_set",
    "content": {
      "states": [
         {
           "name": "Kigali",
           "districts": [
             {
               "name": "Gasabo",
               "wards": [...]
             },
             ...
           ]
         },
         ...
      ]
    }
  }
]

[rowanseymour]

Added in #115