Importers: standardise import file validations
<!--IssueSummary start--> <details> <summary> Everyone can contribute. [Help move this issue forward](https://handbook.gitlab.com/handbook/marketing/developer-relations/contributor-success/community-contributors-workflows/#contributor-links) while earning points, leveling up and collecting rewards. </summary> - [Close this issue](https://contributors.gitlab.com/manage-issue?action=close&projectId=278964&issueIid=452230) </details> <!--IssueSummary end--> Create a reusable component for importers which can perform a consistent set of file validations on import archives (and other files?). - Archive size - Archive type - Archive content
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