Raise a ValidationError for incorrect related slug when importing resources

[AdrianDAlessandro]

Description

This PR prevents incorrect slugs from being ignored when importing to the DB using the Resource classes in io_resources.py

It achieves this by adding a check to the clean method of the ForeignKey and M2M Widget classes. If any of the slugs provided in the import data do not exist, it raises a ValidationError. This error provides a dictionary with the offending field as keys, which renders nicely in the Admin backend.

M2M Example with the following import data

name,description,slug,kind,url,learning_resources
new tool,description,new-tool,tool,,what_is_open_source_software_oss|not-a-slug

ForeignKey Example with the following import data

name,description,slug,language,url,provider
new tool,description,new-tool,en,,not-a-slug

Fixes #630

Type of change

• Documentation (non-breaking change that adds or improves the documentation)
• New feature (non-breaking change which adds functionality)
• Optimization (non-breaking, back-end change that speeds up the code)
• Technical work (non-breaking, change which is work as part of a new feature)
• Bug fix (non-breaking change which fixes an issue)
• Breaking change (whatever its nature)

Key checklist

• All tests pass (eg. python -m pytest)
• The documentation builds and looks OK (eg. mkdocs serve)
• Pre-commit hooks run successfully (eg. pre-commit run --all-files)

Further checks

• Code is commented, particularly in hard-to-understand areas
• Tests added or an issue has been opened to tackle that in the future. (Indicate issue here: # (issue))

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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