Allow plugins to define multiple blueprints

If `IBlueprint.get_blueprint()` returns a list, register the `Blueprint` objects in a loop. Backwards compatible; will still work if the method only returns a single `Blueprint`.

ckan/config/middleware/flask_app.py

@@ -178,7 +178,11 @@ def hello_world_post():
     # Set up each IBlueprint extension as a Flask Blueprint
     for plugin in PluginImplementations(IBlueprint):
         if hasattr(plugin, 'get_blueprint'):
-            app.register_extension_blueprint(plugin.get_blueprint())
+            plugin_blueprints = plugin.get_blueprint()
+            if not isinstance(plugin_blueprints, list):
+                plugin_blueprints = [plugin_blueprints]
+            for blueprint in plugin_blueprints:
+                app.register_extension_blueprint(blueprint)
 
     # Set flask routes in named_routes
     for rule in app.url_map.iter_rules():