Presidential Primary Polling Analysis in Stan

I often use random walk/autoregressive models in my research as a component in time-series analysis, and I wanted to get some more experience fitting them to data. FiveThirtyEight publishes several polling datasets, including polling for the 2020 Democratic presidential primary. I used Stan to fit a Bayesian random walk model to the polling data, which I describe below. The Stan and R code used in this post is available as a Github gist.