modernize relationship configuration between markdown files

[travi]

it looks like there are improved techniques available that weren't when the relationships were originally configured: https://www.gatsbyjs.org/docs/gatsby-config/#mapping-node-types

the current mappings are defined here:

site/gatsby-config.js

Lines 5 to 10 in dd36be7

	mapping: {
	'MarkdownRemark.frontmatter.talk': 'MarkdownRemark.frontmatter.title',
	'MarkdownRemark.frontmatter.speaker': 'MarkdownRemark.frontmatter.name',
	'MarkdownRemark.frontmatter.sponsor': 'MarkdownRemark.frontmatter.name',
	'MarkdownRemark.frontmatter.host': 'MarkdownRemark.frontmatter.location'
	},