DEV: Replace instances of Discourse.base_uri with Discourse.base_path
This is clearer because the base_uri is actually just a path prefix. This continues the work started in 555f467.
Google insists on indexing pages so it can figure out if they
can be removed from the index.
see: https://support.google.com/webmasters/answer/6332384?hl=en
This change ensures the we have special behavior for Googlebot
where we allow indexing, but block the actual indexing via
X-Robots-Tag
