fix: header/footer fragment loading (#13)

[Buuhuu]

This PR fixes the broken footer loading from a given footer field in the page metadata.

It also addresses the use case where the footer or nav are configured as paths only instead of a link, for example when using bulk metadata it seems reasonable to configure footer and nav like

Path	Nav	Footer
/**	/nav	/footer
/de**	/de/nav	/de/footer

• Before: https://main--aem-boilerplate--adobe.hlx.live/
• After: https://fix-header-footer-loading--aem-boilerplate--buuhuu.hlx.live/

[aem-code-sync[bot]]

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