First draft of codespeedization of test/perf

* Change webhook notification URLs to julia.mit.edu (Linux) and criid.ee.washington.edu (OSX)
* Add `codespeed` target to test/perf/Makefile, which will run perfsuite and upload result to hardcoded codespeed installation location
  * Note that `make codespeed` will attempt to `Pkg.add()` the packages `JSON` and `Curl` if not already installed.
  * The sort subtests run a limited subset of 28 tests instead of almost 800
* Annotate all @time macro calls with an extra "Description" field, used to populate the codespeed database for the first time
* Until extra build-time information is provided, expect environment variables such as JULIA_FLAVOR, JULIA_BRANCH, and JULIA_COMMIT_DATE on the command line when `make codespeed` is invoked.