Addons: add X-RTD-Load-Addons-When-Embedded HTTP header

Add an extra HTTP header to decide whether or not force the injection of addons
when the page is embedded (eg. iframe). By default, we are not loading addons if
embedded.

Required by readthedocs/addons#415
Closes readthedocs/addons#412

Author: humitos

readthedocs/projects/models.py

@@ -167,6 +167,10 @@ class AddonsConfig(TimeStampedModel):
         help_text="Enable/Disable all the addons on this project",
     )
 
+    # Whether or not load addons library when the requested page is embedded (e.g. inside an iframe)
+    # https://github.com/readthedocs/addons/pull/415
+    load_when_embedded = models.BooleanField(default=False)
+
     # Analytics
 
     # NOTE: we keep analytics disabled by default to save resources.

readthedocs/proxito/middleware.py

@@ -30,7 +30,7 @@
     unresolver,
 )
 from readthedocs.core.utils import get_cache_tag
-from readthedocs.projects.models import Project
+from readthedocs.projects.models import AddonsConfig
 from readthedocs.proxito.cache import add_cache_tags, cache_response, private_response
 from readthedocs.proxito.redirects import redirect_to_https
 
@@ -283,12 +283,13 @@ def add_hosting_integrations_headers(self, request, response):
         project_slug = getattr(request, "path_project_slug", "")
 
         if project_slug:
-            addons = Project.objects.filter(
-                slug=project_slug, addons__enabled=True
-            ).exists()
+            addons = AddonsConfig.objects.filter(project__slug=project_slug).first()
 
             if addons:
-                response["X-RTD-Force-Addons"] = "true"
+                if addons.enabled:
+                    response["X-RTD-Force-Addons"] = "true"
+                if addons.load_when_embedded:
+                    response["X-RTD-Load-Addons-When-Embedded"] = "true"
 
     def add_cors_headers(self, request, response):
         """