We should improve our env variable names

[infomiho]

We have now:

• REACT_APP_API_URL (client), defines the API url on the client. It uses a legacy prefix from the React Create App days. We should probably go for a tech agnostic prefix e.g. WASP_ and CLIENT_.
• WASP_WEB_CLIENT_URL (server) defines the client URL that's useful for OAuth.
• WASP_SERVER_URL (server) defines the server URL that's useful for OAuth.

We should sync the naming between API_URL and SERVER_URL to reduce the possible confusion, since those two are the same thing (server url).

We could do this issue in concert when we do the env vars validation issue which would require us to list out all of the variables and validate them. #1067