[#74047832] Disable ARR session affinity

TL;DR: This commit disables the `ARRAffinity` cookie which Azure sets to
tell application and request routing to ensure a client's connection to a
website always stays on the same IIS instance (as Azure Web Sites are PaaS
with many IIS backends).

Deploying a `web.config` using Git should override the one which exists in the 
application at the remote end, and is just pushed in. This smooths the deployment
pipeline out a bit, too, hopefully.

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Azure proxies requests through application and request routing (ARR) servers
which sit behind the Azure load balancers. These ARR servers are, in
essence, sticky load balancers meaning that each website sticks to the same
IIS instance. Setting the `ARRAffinity` cookie means that if the cookie exists
then Azure will re-route requests back to the IIS backend originally
responsible for serving the website. Disabling the cookie stops this from
occurring (it's difficult to sort scaling with it enabled, for instance). I believe we're
disabling it to ensure that this app doesn't set any cookies.