Add tooling to support improvements in page load latency

[kevinrobinson]

Spun out from #1386.

Scout and Heroku analytics aren't great for our use case, since the request rate isn't steady and high enough.

A minimal version of this could be reading this out of Heroku logs for analysis, or adding a Rails middleware to write this to a database table (eg, datetime, url, response latency).

A more involved solution might add report to a Graphite instance or something like that, or adding client-side latency monitoring (to measure network latency and JS execution time).