Add token as a parameter

[robinbowes]

I'm not sure what token is, but it seems wrong to read it from the environment in the make_webook_request function.

This change makes token a parameter, which is passed into post_to_webhook.

Would it be possible for documentation to be added for token?

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[robinbowes]

@googlebot I signed it!

[davidstanke]

Good call -- the code feels cleaner this way and I've confirmed that it works. LGTM, thanks!