URL validation implementation needs timeout configuration

[coderabbitai]

There's a potential issue in the url_exists() function in util/lib.py where the lack of a timeout configuration in the requests.head() call could lead to hanging requests.\n\nRequested by @anitacaron.\n\nRelated PR: https://github.com/OBOFoundry/OBO-Dashboard/pull/141\nComment URL: #141 (comment)