[Fixes #14705] Metadata validation: call validator from admin page

[etj]

Fixes #14705

Port of the 5.1.x work (14705_51x_validate_admin): extracts MetadataValidator out of the validate_metadata management command into geonode/metadata/validation/, adds a validate_metadata admin action (reports outcome as admin messages) and a download_validation_report action (full JSON report, uncapped) on DatasetAdmin, DocumentAdmin, MapAdmin and GeoAppAdmin, plus tests for both the validator and the admin actions. Includes the follow-up fix capping schema-error admin messages and pointing at the download action for the rest, instead of risking an unbounded loop against Django's cookie-based message storage.

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