Fix some issues reported by lintian

Fix some issues reported by lintian

• Use versioned copyright format URI. (unversioned-copyright-format-uri)
• Use secure URI in Homepage field. (homepage-field-uses-insecure-uri)
• Bump debhelper from old 11 to 13. (package-uses-old-debhelper-compat-version)
• Set debhelper-compat version in Build-Depends. (uses-debhelper-compat-file)

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Build and test logs for this branch can be found at https://janitor.debian.net/lintian-fixes/pkg/gentlyweb-utils/7dd09801-d925-43dd-b9c6-1302b5566cc8.