Add SENTRY_DOMAIN configuration setting

[sohkai]

See #1457 (comment).

The rationale behind this is a user may want to consume or host the same IPFS content as on *.aragon.org, but not want to provide telemetry back to our Sentry instance.

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There should be an easy, obvious way to match window.location.href with a wildcard rule like *.aragon.org, but I haven't managed to find it yet.