add conditional to check showall is really toggled

I think this one conditional eliminates a huge amount of slowdowns. On initial page load, showall is not set, but an XHR request still fires to /mergedids. This conditional prevents that.

The same request fires on requests to /alerts.html?rev=foobar, which most definitely does not need all 127 days worth of raw json to show a single alert, so this conditional stops that request too.

Thoughts? I hope that I understand how and why showall is used - my understanding is that it is mainly a way to toggle showing expired alerts in the main /alerts.html view.