Fix supporting document reference

[AidenEscamilla]

What

1. Model to act as intermediary / Join Table for the many-to-many relation between job_applications and supporting_documents
2. Migration for the described model
3. Existing model updates to work with the new intermediary
4. Existing form and display updates to work with new intermediary

Why

Turns out using has_many and/or 'belongs_to' creates an unwanted relationship between the applications and documents. The man-in-the-middle class allows for a common document to now be shared with multiple applications.

The supporting_document form now uses check boxes to select multiple applications. The model #show also displays all the applications it's attached to.

The job_application #show continues to displays all the documents associated and now works with the intermediary linker.

TODO / Next Steps

• Validations
• Double check destroy works correctly
• Move linker.create! Out of the controller and into a Form.object for supporting_document
• Factory bot for supporting_document
• document_linker specs