Add Sentry error reporting

[hwittenborn]

The current plan is to add Sentry reporting to builds of Mist so that errors can be automatically diagnosed when they happen. The current plan is that any panics will be automatically captured, and that's all (though it may be expanded upon in the future).

On first startup, we should prompt the user if they'd like to send crash report data. This needs to be opt-in, so default to sending nothing. The result will be written to a user-specific config for Mist, so we need to work on making that config file first.