Fix 'url' property in search results

When documents are returned from elasticsearch they don't have an '_id'
property, but they do have an `id` property, whereas mongoose documents
are the opposite. So we check for both properties when constructing the
'url' property.
Fix issue with embedded document 'url' property changing

When the apiLinks middleware sets the baseUrl and apiBaseUrl
properties on req.collection.schema it only affected the results
for the current collection. This changes it to loop through all the
popolo schemas and set the base urls for each one.
Merge pull request #30 from mysociety/fix-api-links

API links fixes