Add support for client-side redirects in production

[KyleAMathews]

/cc @bvaughn

This PR also adds a flag to createRedirect "redirectInBrowser" which a user
must opt into for the redirect to be run in the browser. This is done as there
are many scenarios where people building sites with Gatsby might have hundreds
or thousands of redirects from legacy versions of the site, none of which
we'd want loaded into the browser.

[gatsbybot]

Deploy preview ready!

Built with commit 71f8f20

https://deploy-preview-2227--gatsbygram.netlify.com

[bvaughn]

Thanks for the cc! Will try to find time to look at this soon, but don't have it at the moment.

WRT #2226, I'm initially confused that the current behavior (in master) is different based on the absence/presence of a trailing slash though. Note to myself to figure out why that is when I later look at this PR.

Edit: Disregard. It looks like the trailing slash "fix" didn't work on prod after all, even though it did on the preview-built version of the site. I believe this was because we were using an <a> tag in one place and a Gatsby <Link> tag in another.

[bvaughn]

This looks good to me, at a glance. Did not test it yet though. Will be happy to test it when a new version is released, provided #2225 does not block me from upgrading.

[bvaughn]

FYI, I updated the React site to gatsby@1.9.44 and changing our gatsby-node.js to:

// Blog landing page should always show the most recent blog entry.
createRedirect({
  fromPath: '/blog/',
  redirectInBrowser: true,
  toPath: newestBlogNode.fields.slug,
});

Works like a charm 😄 Thanks!