refs https://github.com/TryGhost/Team/issues/1284
When you create a new post with a tag slug that contains spaces, those spaces will get replaced by dashes. But instead of reusing an existing tag, a new tag is always created.
- New tag slugs are cleaned up before matching with existing tags in the Post model onSaving method
- Cleaned up multiple loops in onSaving of Post model
- Cleaned up syntax when cleaning up tag slug
- Added tests for slugs with spaces
- Added test for too long tag slug causing duplication