Convert addresses pointing to localhost in status.

Until now, targets pointing to localhost in the status view are linked to localhost, so you can't follow those links by clicking on them.
This change converts the links to point to the hostname of the prometheus server.

Before:
<a href="http://localhost:9090/metrics.json">http://localhost:9090/metrics.json</a>

After:
<a href="http://hostname-of-prometheus-server:9090/metrics.json">http://localhost:9090/metrics.json</a>
Merge pull request #128 from prometheus/feature/convert-host-relative-links

Convert addresses pointing to localhost in status.