change `past-speakers` route to just `speakers`

[jgbowser]

closes #158

NOTE: Do not merge this in until the the internal link for "Speakers" has been updated and published in Sanity or else the Speakers nav link will 404. I'd recommend doing this in the late evening to reduce the odds of anybody encountering this in case there is a brief window where the app and sanity are out of sync during deploy

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