Enable finding existing hashtags case-insensitive

We should store hashtags case-sensitive, but ensures that an existing
hashtag with different case are found and re-used. for example,
an existing #BookWyrm hashtag will be found and used even if the
status content is using #bookwyrm.

bookwyrm/activitypub/note.py

@@ -65,6 +65,7 @@ class Note(ActivityObject):
                 rf'(<a href=")[^"]*(" data-mention="hashtag">{hashtag.name}</a>)',
                 rf"\1{hashtag.remote_id}\2",
                 instance.content,
+                flags=re.IGNORECASE,
             )
             if instance.content != updated_content:
                 instance.content = updated_content