Refactor Data Feed settings

[rafalcymerys]

This PR introduces some changes to the Data Feeds feature:

• Renamed DataFeedSetting to DataFeed -
• Renamed enabled field to active, to keep it consistent with other entities in the database
• Renamed spree_store_id to store_id, to keep it consistent with other entities in the database, and avoid having to define foreign_key explicitly
• Renamed uuid to slug
• Added indexes that will improve the lookup process
• Changed the uuid/slug generation process - it will be generated every time an empty slug field is provided (even during update). Also, when a custom slug is provided, it will convert it to a value that's safe for URL.

[viezly]

This pull request is split into 3 parts for easier review.
👀 Review pull request on Viezly

Changed files are located in these folders:

• api/app
• api/config
• core/app
• core/db/migrate
• core/lib/spree
• sample/db/samples
• sample/lib