Change how blog post "path" frontmatter works

[benslv]

Currently, a blog post's frontmatter looks a little bit like this:

---
path: "/blog/site-welcome"
date: "2020-04-16"
author: "Ben Silverman"
title: "Welcome to our new website!"
tags: ["post"]
---

with path telling Gatsby at what URL to put the blog post once it's been generated.

I don't really like how the /path/ is included in there, because it means people could put posts at any URL they want on the site, rather than as a sub-page of the blog. I think it should be renamed to be the "slug" of the post (i.e. a unique, kebab-case string which identifies the post and will be used in the URL.

TL;DR: Gatsby should still place all blog posts at /path/{slug} but the /path/ part should be moved to gatsby-node.js and handled at build time.