Remove alternate title functionality

This removes the functionality to create and edit alternate titles for editions within publisher. The intention is to get this change deployed first before making an export of the data before it's finally removed from the respective dependencies (govuk_content_models) and database.

Story: https://www.agileplannerapp.com/boards/173808/cards/8145

The second commit in this PR is to add a respective rake task to export this data once this change is deployed.
`Edition.where(:alternative_title.nin => ["", nil])` would complete much faster. I've not checked recently how many `Edition`s we have on production, but changing this will help if the number is huge.