Display number of clients served on icon badge

People like to see this number, but currently they need to open the popup to check it out: https://gitlab.torproject.org/tpo/anti-censorship/pluggable-transports/snowflake-webext/-/blob/2ba8f859afbfb5259c6c3a830eb94fb3d536f351/static/popup.js#L70

Although seeing it go down is a bit weird, so maybe show the amount of clients since the extension was installed, instead of in the last 24h. Although the badge is gonna overflow after 10000 clients (~ a year)). Or only show it for a short period of time when the count goes up. Or maybe it's not that weird.

There is browser.browserAction.setBadgeText and browser.browserAction.setBadgeBackgroundColor API for this.