Validate hostnames instead of domain names

[jasikpark]

Changes:

• updates the hostname validator to accept hostnames in addition to domain names
• remaps the input to lower case to indicate the case insensitive DNS matching
• uses parse-domain to provide useful errors
• npm run format

test w/ https://deploy-preview-87--nebula-docs-dn.netlify.app/docs/guides/using-lighthouse-dns/#hostname-validity

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[jasikpark]

I should duplicate

/* Prevent ios from "zooming in" */
.navbar__search-input {
  font-size: 1rem;
}