dev/core#2998 - Add Standalone uf

In standalone cms, the getting started dashlet gives an error 500, which is confusing at first because it seems like it's a local 500, but it's coming from alert.civicrm.org because it doesn't recognize standalone.

I'm not sure if this will start to appear on stats.civicrm.org and whether that's good or bad. There's (probably) only two sites right now, and both are running dev versions so shouldn't appear.