Deliver content of mirrored page via API
So far the content of the mirrored page isn't delivered via the API

Steps to reproduce:

1. Add mirrored content to page
2. Save
3. Access API (for example http://0.0.0.0:8000/api/augsburg/de-de/pages/)
diff --git a/backend/api/v3/pages.py b/backend/api/v3/pages.py
index 77eae21ba..b884fb58e 100644
--- a/backend/api/v3/pages.py
+++ b/backend/api/v3/pages.py
@@ -23,7 +23,7 @@ def transform_page(page_translation):
         'title': page_translation.title,
         'modified_gmt': page_translation.last_updated,
         'excerpt': page_translation.text,
-        'content': page_translation.text,
+        'content': page_translation.combined_text,
         'parent': parent,
         'order': page_translation.page.lft, # use left edge indicator of mptt model for order
         'available_languages': page_translation.available_languages,
diff --git a/backend/cms/models/pages/page.py b/backend/cms/models/pages/page.py
index 528581adb..11b4abf08 100644
--- a/backend/cms/models/pages/page.py
+++ b/backend/cms/models/pages/page.py
@@ -88,7 +88,9 @@ class Page(MPTTModel):
         """
         This content needs to be added when delivering content to end users
         """
-        return self.mirrored_page.get_translation(language_code).text
+        if self.mirrored_page:
+            return self.mirrored_page.get_translation(language_code).text
+        return None
 
     def get_absolute_url(self):
         return reverse('edit_page', kwargs={
diff --git a/backend/cms/models/pages/page_translation.py b/backend/cms/models/pages/page_translation.py
index 204b8406b..65eee13a8 100644
--- a/backend/cms/models/pages/page_translation.py
+++ b/backend/cms/models/pages/page_translation.py
@@ -130,9 +130,12 @@ class PageTranslation(models.Model):
         """
         Combines the text from the PageTranslation with the text from the mirrored page.
         """
+        attached_text = self.page.get_mirrored_text(self.language.code)
+        if attached_text is None:
+            return self.text
         if self.page.mirrored_page_first:
-            return self.page.get_mirrored_text(self.language.code) + self.text
-        return self.text + self.page.get_mirrored_text(self.language.code)
+            return attached_text + self.text
+        return self.text + attached_text
 
     def __str__(self):
         if self.id:
['backend/cms/models/pages/page_translation.py', 'backend/api/v3/pages.py', 'backend/cms/models/pages/page.py']