Use Logger.warning instead of Logger.warn

Logger.warn is deprecated
https://docs.python.org/3/library/logging.html#logging.Logger.warning
Drop reference to django mptt
Fix migrations name
Merge pull request #3530 from yakky/feature/drop_mptt_docs

Drop reference to django mptt
Merge pull request #3531 from yakky/feature/fix_migration_name

Fix migrations name
Merge pull request #3528 from vstoykov/logger-warnings

Use Logger.warning instead of Logger.warn