Geo secondary proxying: serve assets locally

What does this MR do and why?

As Geo sites are required to be on the same version as the primary, the assets should be the same, and at the same path, therefore we can speed this up by serving them from the local secondary when proxying is in use.

Related to #347120 (closed)

How to set up and validate locally

With GDK + Geo set up (or on a GET instance) using secondary proxying:

1. Tail the logs on the primary
2. Make a request through the secondary (hard refresh on the homepage or a specific page), notice assets/* paths being requested
3. On the secondary, cd workhorse && make && gdk restart gitlab-workhorse
4. Hard refresh again, notice page loading fine, no assets requested in the primary logs

MR acceptance checklist

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