{{#unless node.fields.nice_url}}
<link rel="canonical" href="{{node.path}}" />
{{else}}
<link rel="canonical" href="{{node.fields.nice_url}}" />
<link rel="alternate" href="{{node.path}}" />
{{/unless}}
{{#each node.fields.alternate_urls}}
<link rel="alternate" href="{{this}}" />
{{/each}}