Lag chart

This branch adds a lag chart for debugging server/connection performance issues.

The chart works by measuring the time difference between the newest usercommand on the stack, and the usercommand most recently confirmed by the server.

This pull request depends on health and splash, although it probably doesn't have to. When I started it, I was expecting it to depend more on them than ended up being the case, and I didn't really see a reason to rebase (and make other merges harder) unless one of the other pull requests gets rejected/requires significant changes.

The charts are in a weird position because the are designed to fit directly under the chrome built in FPS counter (which you can enable in chrome://flags/).