Add method to migrate existing pages to another locale

Relates to MS-749 #complete

Add docstring for new migrate method

server/core/migrate_i18n_pages.py

@@ -0,0 +1,53 @@
+from wagtail.models import Locale, Page
+
+from books.models.module import Module
+from books.models.topic import Topic
+
+from core.logger import get_logger
+
+logger = get_logger(__name__)
+
+
+# todo: make this a django command
+# todo: add language to translate to as an argument
+# todo: write a test maybe
+def migrate_i18n(slug: str, original_slug: str):
+    """
+    Migrate a module and its children to a new locale and links it to
+    its translation in the original locale.
+
+    @param slug: the slug of the module to migrate
+    @param original_slug: the slug of the module in the original locale
+    """
+    module = Module.objects.get(slug=slug)
+    original = Module.objects.get(slug=original_slug)
+    en = Locale.objects.get(language_code="en")
+    logger.debug("starting this")
+    topic: Topic = module.get_parent()
+    try:
+        logger.debug("getting translation")
+        en_topic = topic.get_translation(en)
+    except Page.DoesNotExist:
+        logger.debug("creating translation")
+        en_topic = topic.copy_for_translation(en)
+        revision = en_topic.latest_revision
+        en_topic.publish(revision)
+
+    logger.debug(f"new topic {en_topic.pk}")
+    module.locale = en
+    if original is not None:
+        translation_key = original.translation_key
+        module.translation_key = translation_key
+    module.save()
+    chapters = module.get_children()
+    for chapter in chapters:
+        chapter.locale = en
+        chapter.save()
+        content_blocks = chapter.get_children()
+        for content_block in content_blocks:
+            content_block.locale = en
+            content_block.save()
+
+    module.move(en_topic, "last-child")
+
+    print(f"go {module.title} {module.locale.language_code}")