[#331] Add caching to map resources

Add caching of the serialized data for ProjectMapLocationResource and
write OrganisationMapLocationResource using the same pattern.

Add the base class CachedMapResource used by ProjectMapLocationResource
and OrganisationMapLocationResource.

Add MapLocationResource and use as a base class for ProejctMapLocationResource and OrganisationMapLocationResource.
[#331] Re-instate API driven global maps

Remove the static imgages from the global maps templates and use the
template tag for real maps instead.

Set the limit to 500 for the maps API calls in akvo-maps.js.

Fix HOST in the map.py templatetag code.