Implement webhook to categorize browser-*, os-*, form-* with labels

[karlcow]

The webhook needs to label for

• browser-[name] or browser-fixme for unknown browsers
• os-{windows,macos,linux,android,ios}
• form-{mobile,desktop,tablet}
• version-nnn

Context, History

• [Brainstorming] Re-considering the way we apply browser-labels #2372
• better labeling for the project #997
• Filtering out (or not) the version-less browser name in tagging #1949
• Investigate "browser-firefox-mobile-nightly-59.0a1" label on issue #1971

[miketaylr]

Per @karlcow's request, adding a note here that we should attempt to not break engine-gecko when we do this (possibly by re-writing how that is done).

Unrelated, we probably also want form-vr too!