* Updates Fenix taskcluster tasks to support beta release
* Throw error if -PversionName isn't set for release builds
* Uses beta secrets for beta
* Improves nightly and beta treeherder symbols
* greenfield (pure Fenix without anything added)
* firefoxNightly (To replace Firefox Nightly)
* firefoxBeta (To replace Firefox Beta)
* firefoxRelease (To replace Firefox Release)
For now it will be a bit annoying to have all those flavors. But once the transition has been
successfully completed we can start to tear them down again.
Once GeckoView universal builds are available we can get rid of the "abi" dimension.
If this is going to slow down the CI process then I might look into parallelizing the build
or potentially introducing a taskcluster CI setup (if that makes sense).