Make the perf dashboard usable

[philbooth]

@rfk suggested adding some commentary to and re-organising the charts on the perf dashboard. At the moment, all the definitions for what things mean are inside my head, which is not very helpful.

[philbooth]

It took much longer than I thought it would, but I've now done this to my satisfaction:

https://sql.telemetry.mozilla.org/dashboard/fxa-performance

That's the best I can do while still preserving the chart order that I get most benefit from, closing this.