Change Document publication_date column type from :date to :string

[sarastanton]

What this PR does:

• Change publication_date field for Document records from date to string
• Existing date data will be converted to string in 'YYYY-MM-DD' format; no change needed for existing data

Steps to test:

1. Navigate to Documents index (/documents) and click on 'Edit' button for any given document
2. Update 'Publication field' date to anything (i.e. "Early 1800s", "1826-04-23", "Victorian Era", "1826", etc.) and save changes
3. Navigate away and back to same Document record - observe that updated 'Publication field' value persists

Visual change(s): N/A

[sarastanton]

This migration will automatically convert date attributes to string attributes in 'YYYY-MM-DD' format