Add Drafts table to database

[seesarahcode]

• Create Drafts table
• Add publication_id column - integer
• Add component_id column - integer
• Add label column - string
• Add due_date column - datetime
• Add is_final column - boolean
• Add required column - boolean
• Add reminder_frequency column - string
• Add validation for publication_id column (presence)
• In Publications table add: has_many :drafts
• Drafts table should: belongs_to :publication
• In Components table add: has_many :drafts
• Drafts table should: belongs_to :components
• Drafts table should: has_many :notifications
• In Notifications table add: belongs_to :draft
• Create factories
• Write passing unit tests
• Create seed data