Expose canonical URLs to improve searchability in web crawlers

[DougReeder]

Is your feature request related to a problem? Please describe.
A number of the URLs in Hubs, including the root URL \, are also served at other URLs. Web crawlers like Google often decline to index pages where they can't determine the canonical URL. They may also conclude different instances of Hubs are actually the same. This makes it unlikely that the URLs will show up in search results, or they may show up when they shouldn't.

Describe the solution you'd like
Either

1. To the page header, add a rel="canonical" link
2. To the HTTP header, add a rel="canonical" header

Describe alternatives you've considered
A sitemap would provide a weaker signal of canonical paths.

Additional context
Here, Google has confused the signin page of my instance, with that of hubs.xrspaces.de: