make app x-forwarded-prefix aware

[dennyh89]

I wanted to deploy the app in a multizone mesh and behind a delegated gateway that uses path-based routing with strip_path, which leads to issues that the frontend is trying to call "/increment" without the prefix attached.

I have no experience in nodejs so i am open to discuss the changes, rework the contribution or just even learn how this could have been achieved more easily.

this commit adds the prefix to each link in the index.htlm and uses ejs to accomplish that.

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