Add pathPrefix support to manifest plugin

[canastro]

This fixes #2078 adding pathPrefix support to the gatsby-plugin-manifest.

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[KyleAMathews]

Nice, thanks!

[jlengstorf]

Hiya @canastro! 👋

This is definitely late, but on behalf of the entire Gatsby community, I wanted to say thank you for being here.

Gatsby is built by awesome people like you. Let us say “thanks” in two ways:

1. We’d like to send you some Gatsby swag. As a token of our appreciation, you can go to the Gatsby Swag Store and log in with your GitHub account to get a coupon code good for one free piece of swag. (We’ve got t-shirts and hats, plus some socks that are really razzing our berries right now.)
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If you have questions, please don’t hesitate to reach out to us: tweet at @gatsbyjs and we’ll come a-runnin’.

Thanks again! 💪💜

[canastro]

Awesome, thanks! Socks on their way :D