?_labels= and ?_label=COL to expand foreign keys in JSON/CSV

These new querystring arguments can be used to request expanded foreign keys
in both JSON and CSV formats.

?_labels=on turns on expansions for ALL foreign key columns

?_label=COLUMN1&_label=COLUMN2 can be used to pick specific columns to expand

e.g. `Street_Tree_List.json?_label=qSpecies&_label=qLegalStatus`

    {
        "rowid": 233,
        "TreeID": 121240,
        "qLegalStatus": {
            "value" 2,
            "label": "Private"
        }
        "qSpecies": {
            "value": 16,
            "label": "Sycamore"
        }
        "qAddress": "91 Commonwealth Ave",
        ...
    }

The labels option also works for the HTML and CSV views.

HTML defaults to `?_labels=on`, so if you pass `?_labels=off` you can disable
foreign key expansion entirely - or you can use `?_label=COLUMN` to request
just specific columns.

If you expand labels on CSV you get additional columns in the output:

`/Street_Tree_List.csv?_label=qLegalStatus`

    rowid,TreeID,qLegalStatus,qLegalStatus_label...
    1,141565,1,Permitted Site...
    2,232565,2,Undocumented...

I also refactored the existing foreign key expansion code.

Closes #233. Refs #266.

tests/test_api.py

@@ -1096,6 +1096,52 @@ def test_suggest_facets_off():
         ).json["suggested_facets"]
 
 
+def test_expand_labels(app_client):
+    response = app_client.get(
+        "/test_tables/facetable.json?_shape=object&_labels=1&_size=2"
+        "&neighborhood__contains=c"
+    )
+    assert {
+        "2": {
+            "pk": 2,
+            "planet_int": 1,
+            "state": "CA",
+            "city_id": {
+                "value": 1,
+                "label": "San Francisco"
+            },
+            "neighborhood": "Dogpatch"
+        },
+        "13": {
+            "pk": 13,
+            "planet_int": 1,
+            "state": "MI",
+            "city_id": {
+                "value": 3,
+                "label": "Detroit"
+            },
+            "neighborhood": "Corktown"
+        }
+    } == response.json
+
+
+def test_expand_label(app_client):
+    response = app_client.get(
+        "/test_tables/foreign_key_references.json?_shape=object"
+        "&_label=foreign_key_with_label"
+    )
+    assert {
+        "1": {
+            "pk": "1",
+            "foreign_key_with_label": {
+                "value": "1",
+                "label": "hello"
+            },
+            "foreign_key_with_no_label": "1"
+        }
+    } == response.json
+
+
 @pytest.mark.parametrize('path,expected_cache_control', [
     ("/test_tables/facetable.json", "max-age=31536000"),
     ("/test_tables/facetable.json?_ttl=invalid", "max-age=31536000"),