Deploy to a cluster link in search performance warning notice is relative, not absolute

Created by: janisz

Steps to reproduce:

1. Go to https://demo.sgdev.org
2. Search for (hunk|repository)resolver index:only timeout:1000s
3. Notice Search performance warning shown. Click on deploy to a cluster link.
4. Taken to repo 404 page as the link was /https://docs.sourcegraph.com/admin/install/cluster.

Refs: https://github.com/sourcegraph/sourcegraph/issues/4928