Delete prefix requirement for appIndex redirect.

[btelles]

See issue #1334 for context.

This removes the requirement that the devserver see the appIndex relative path at the beginning of the url path in order for the devserver to redirect to the index page. As a developer, I'd expect any page that does not match a URL redirect to the index.html file. This is especially helpful for monorepos where the appIndex file ends up pointing to /packages/example/dev/index.html.

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Latest commit: 5ed05faca14a59d4b4ee763b10c32f08eec74a6c

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[btelles]

Gentle ping regarding this PR. Anyone willing to merge this?

[btelles]

Is there anything I can do to bring attention to this pull request? I think it's relatively simple and would fix a problem for at least two users.