Fix the Firefox extension connection status indicator

Created by: chrismwendt

Fixes https://github.com/sourcegraph/sourcegraph/issues/4114 by reverting:

• https://github.com/sourcegraph/sourcegraph/pull/3933 which upgraded to core-js@3
• https://github.com/sourcegraph/sourcegraph/pull/3958 which replaced the deprecated @babel/polyfill with core-js/stable (which only exists in core-js@3)

I didn't find a solution with core-js@3 within ~30 minutes, so I'm proposing this PR to at least fix the Firefox extension now while we find another solution.

This can be caught by puppeteer-firefox (originally proposed in https://sourcegraph.slack.com/archives/CHXHX7XAS/p1557440150011000)