F fix url formatting

Summary

Part of https://gitlab.indiscale.com/caosdb/customers/geomar/management/-/issues/302. Use urllib.parse.join instead of string concatenation for link generation.

Focus

Nothing new, just use built-in functions to prevent missing or extra / and the like.

Test Environment

Run with Geomar server profile, set crawler in profile.yml to this branch, upload some samples and look into the sendmail_to_file output in /tmp/mail to verify that the URIs are generated correctly.

Also, new unit test.

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Check List for the Reviewer

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