Merge lp://staging/~knitzsche/scope-aggregator/fixes-fallback-bug-1599948 into lp://staging/scope-aggregator

When testing photos scope I found an issue that this commit fixes.

Please review. The key change is line 1192.

DETAILS

The issue I noticed when testing this MP in photos-scope is that the art for
Douban LOGIN result did not display in photos agg scope.

I found that the fallback code (check_result_fallbacks()) was not called
for Duoban in the login case.

The general case I found is: For scopes that are declared in child_scopes.json
using the "scope" key, the check_results_callbacks() method was not called.

(Please note that fallbacks code/feature is only currently implemented
for scopes that use a renderer defined in common_templates.)

Teseting. Modify photos scope child_scopes.json to only this for Duoban:

        "scope": {
            "_category_title": "My Douban Photos",
            "id": "com.canonical.scopes.douban_doubanalbum",
            "local_id": "douban",
            "result_category_id_to_common_template": [
              {
                "result_category_id": "login",
                "common_template": "not_logged_in"
              }
            ],
            "renderer_common_id": "normal_renderer"
        }

This ^ means:
 * if Douban result category is "login": use common_template "not logged_in".
    This template uses a fallback:
            "id": "not_logged_in",
            "fallbacks":
            [
                {
                    "key": "mascot",
                    "fields":
                    [
                        {"field": "mascot"},
                        {"field": "art"}
                    ]
                }
            ],
    [...]
    Result is that Douban's login result which provides "art" is converted to
    "mascot".
* A result with any other category uses "normal_renderer". This does not use a
  fallback and expects the result to contain 'art".