Cut new FxA branch from current master, then make master branch the default, cut from latest job-dsl branch

[stephendonner]

I cleared the gist of the following with @philbooth (thx for being so accommodating, Phil!):

• [ ] Move current master branch to something like an "FxA" branch (or all lowercase)
• [ ] Fold current job-dsl branch into newly-available master branch
• [ ] Make sure to update all relevant docs/pointers, as well as...
• [ ] Jenkins: we need to resurrect and make sure that https://qa-preprod-master.fxtest.jenkins.stage.mozaws.net/job/wpt-api.fxa-perf/ continues to run, using the new FxA branch, and for the foreseeable future, continues to send to and visualize test-run data/metrics through DataDog
• [ ] Switch back to `the ``masterbranch in @dependabot[bot] , and see if we can add the newly-createdFxA``` branch, as well, for coverage

/cc @philbooth @davehunt

[davehunt]

I'd prefer to support both FxA and top sites requirements from the master branch. Can you provide an overview of the differences? From a quick look, it's the URL, locations (just Firefox), the JSON filename, and the names of the metrics in Datadog.