Require TLS v1.2 for viewers

[chrsmith]

We'd like to require TLS v1.2 for viewers of https://get.pulumi.com. We initially did this in #32, but later rolled that back in #35 because it causes problems for PowerShell.

However, since then it seems that it's easier for PowerShell to use TLS v1.2 and so we should look into revisiting this.

Relevant links:

• https://blog.pauby.com/post/force-powershell-to-use-tls-1-2/
• https://docs.microsoft.com/en-us/security/engineering/solving-tls1-problem#update-windows-powershell-scripts-or-related-registry-settings