refs TryGhost/Team#1513
- nullable `sender_name` allows us to use auto fallback of site title for sender name without setting any explicit value for it.
refs https://github.com/TryGhost/Team/issues/1500
The newsletter table schema has bunch of changes to go through for new and existing columns, this consolidates the schema changes into a single re-create table migration that drops and adds the newsletter table with correct schema. The table re-create migration needs to run before any of the tables using newsletter as foreign key. The changes include -
- new columns for design related fields
- new slug column for filtering
- unique constraint to `name` column
- remove `default` column (noops the existing default column migration)
- `sender_reply_to` has a default of newsletter and a validation of ['newsletter', 'support']
- updated default values for `subscribe_on_signup` and `recipient_filter`
