[Critical] Make sure the devServer indexPage is only used in dev mode

[ia3andy]

Fixes #603

This is a big issue which could affect production on different kind of apps (not only with Next), it should be backported.

To mitigate this issue:

• if not set (Optional), the dev server index page name is not used, only Next framework automatically overrides it (leading to the production startup failing).
• If set (this is a new and not very commonly used config) , then currently the dev server index page name would be used in production leading to either a startup failing or a 404 on the index page