While we ping some search engines (currently, only Google) when
generating the sitemap files, we weren't telling search engines
accessing through the `robots.txt` file where to find the sitemap. Now
we're doing so, using the right sitemap file for the right tenant.
When administrators disabled features and users tried to access them
with the browser, we were responding with a 500 "Internal Server Error"
page, which in my humble opinion was incorrect. There was no error at
all; the server worked exactly as expected.
I think a 403 "Forbidden" code is better; since that feature is
disabled, we're refusing to let users access it.
We could also respond with a 404 "Not found", although I wonder whether
that'll be confusing when administrators temporarily or accidentally
disable a feature.
A similar thing might happen if we responded with a 410 "Gone" code.
Actually this case might be more confusing since users aren't that
familiar with this code.
In any case, all these options are better than the 500 error.
