environment variable to set TorBrowser homepage

Tor Browser's homepage is currently hardcoded in (aboutTor.js).

kAboutTorURL = "chrome://torbutton/content/aboutTor/aboutTor.xhtml";

It would be nice if there was an TOR_HOMEPAGE environment variable to configure this. Would be useful for (linux) distributions that want to set a divergent default.

I am going to submit a git branch that adds this feature soon.

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