Add mapping and mapping field models

[caitmich]

Summary

With the move to db-backed mappings, we need to add mappings and mapping fields tables.
This PR:

• Backports the addition of mapping and mapping_field tables from Pro
• Removes the destination presence validation on mappings model
• Adds a case_sensitive: false uniqueness validation to mapping's destination and mapping_field's destination_field

Thanks for contributing to Dradis!

I assign all rights, including copyright, to any future Dradis
work by myself to Security Roots.

Check List

• Added a CHANGELOG entry