Fetch, store, and compare (fail/alert, if mis-matched?) Firefox versions between WPT and product details API/JSON

Fetch, store, and compare the following:

1. [product-details] FIREFOX_NIGHTLY which is a string like "66.0a1" (https://product-details.mozilla.org/1.0/firefox_versions.json) in our Product Details JSON/API
2. [product-details] LATEST_FIREFOX_VERSION which is a string like "64.0" (NOTE: we'll need to be careful about allowing for point releases in the version string...) (https://product-details.mozilla.org/1.0/firefox_versions.json)
3. browser_version which is also a string like "66.0a1" (https://www.webpagetest.org/result/181218_4N_5791672a7f164f359dc57a8e78e90a60/?f=json)

And see Patrick Meenan's comment from https://github.com/WPO-Foundation/wptagent/issues/216#issuecomment-439713162:

I took the opportunity to just build a new image off of 18.04.

It has all of the correct current browsers and should update fine but it's worth keeping a track of the > "browser_version" metric from your tests to make sure it is updating when you release a new major > version to Nightly.