[6.x] Digital Products

[duncanmcclean]

This pull request brings the functionality from the Digital Products addon into Simple Commerce Core.

It's being moved into Core so it's easier to maintain alongside other Simple Commerce changes. It should also mean it's more likely for any feature requests to actually be worked on 😆

To Do

• [x] Add some way of toggling whether the digital products functionality is active or not (eg. whether the fields get ensured or not)
• [x] Create an upgrade script to...
  • [x] Migrate product_type field
  • [x] Remove Digital Product fields from blueprints (it'll be re-added in the new place)
• [x] Update default blueprint
  • [x] Update default product blueprint in starter kit
• [x] Add notes to the upgrade guide
  • The notification class will need to be changed.
  • Updated URLs (links in emails will 404)
  • Download History is now always enabled. The config setting for it has been removed.
• [x] Review & make edits to the existing Digital Products documentation
• [x] Do any necessary refactoring

[github-actions[bot]]

Released as part of v6.0.0.