Allow duplicate titles on section import

Added an option to TitleHandler(ColumnHandler) to allow import
collisions on titles. Option is initialized to default to False in
order to prevent this from impacting other imports.
Warn on duplicate titles

Instead of just allowing them to go by without any notice at all,
duplicate titles will now trigger a warning and the import will be
allowed.
Correct section title uniqueness on import

- Sections must be unique globally according to slug
- Unique by title within the parent directive
- Unique by title within the current import
Move row handler.
Check for scoped collsions only.
Merge pull request #1474 from reciprocity/feature/allow-duplicate-titles-on-section-import3

Feature/allow duplicate titles on section import3