Introduce .env.client.production file

[Martinsos]

They right now have to provide production env vars manually to the build command for the client, like REACT_APP_SOME_VAR_NAME=somevalue npm run build.

This can get tedious with more vars, and they will right now likely want to instead have a small script or something that calls the build command with these set for them, and will likely want to commit this to git (since there are no secrets on client anyway).

We could offer more structure / guidance here by instead of having just .env.client, also having two additional files: .env.client.production, and .env.client.local.
.env.client and .env.client.production would be versioned, .env.client.local would not and would serve as a way to override stuff specifically for your local machine.
.env.client could even be renamed to .env.client.dev, but then we would also want to rename .env.client.local to .env.client.dev.local hah.

Btw I see Next has something very similar:

Here is also relevant convo on Discord: https://discord.com/channels/686873244791210014/1204701955679854602/1228366318525218857 .

[Martinsos]

Related to #175

[Martinsos]

+1 for .env.client.dev, people to make it clear these are just for development.