Expiration of outdated images in the database

[ChariniNana]

Fixes

Fixes #482 by @ChariniNana

Description

This pull request handles the image expiration logic, where the updated_on value associated with an image is older than what it can be as per the re-ingestion logic of the corresponding provider.

Technical details

We specify the oldest possible value the updated_on attribute can take per each provider (based on the re-ingestion strategy) in a dictionary. If certain images in the database are deemed to be outdated as per this logic, we set the corresponding removed_from_source value to True.

Tests

The test_image_expiration function in the test_sql test suite checks whether the image expiration logic works as expected.

Checklist

• My pull request has a descriptive title (not a vague title like Update index.md).
• My pull request targets the default branch of the repository (main or master).
• My commit messages follow best practices.
• My code follows the established code style of the repository.
• I added tests for the changes I made (if applicable).
• I added or updated documentation (if applicable).
• I tried running the project locally and verified that there are no
  visible errors.

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[mathemancer]

I made one stylistic suggestion.

Overall, this looks great though, thank you! I'm really excited to have this functionality.

[mathemancer]

Looks great, thank you!