Set up MSI installer

[ewjordan]

This will not work as-is - basically, the steps are:

1. Boot the electronuserland/builder:wine-mono Docker container with the project directory mapped, as explained at https://www.electron.build/multi-platform-build
2. Update yarn on the container
3. Locally, copy the signing .p12 file into the project directory
4. Set WIN_CSC_LINK on the container to the location of the .p12 file
5. export USE_SYSTEM_SIGNCODE=false
6. export SIGNTOOL_PATH=/project/config/osslsigncode
7. Install sudo (via apt-get)
8. sudo yarn win to build

Internal to Code.org, there are more details at https://docs.google.com/document/d/1qYETgpZuYiubKVsRprgGzlj6kEb9W7Xc1Gt6as8v3sA/

We have decided to postpone automating this until we verify that this build works for people, so I'll be leaving this PR open.