Add Submissions table to database

[seesarahcode]

• Create Submissions table
• Add draft_id column - integer
• Add user_id column - integer
• Add comments column - string
• Add validation for draft_id column (presence)
• Drafts table should: has_many :submissions
• In Submissions table add: belongs_to :draft
• Notifications table should: belongs_to :submission
• In Submissions table add: has_many :notifications
• Create factories
• Write passing unit tests
• Create seed data