UTC timestamps everywhere; fix sitemaps (ref #7657)

* `ingestDate` and dashboard doc timestamps are now in UTC with timezone
* Sitemaps require `<lastmod>` dates to be in W3C Datetime format
* Updates JSON Schema to check if `ingestDate` is RFC 3339 compliant
* Use `ingestDate` from items to populate `<lastmod>`
* Move configuration loading logic outside functions
* Add configuration option for local sitemap path

Note: RFC 3339 is a subset of W3C Datetime/ISO 8601.
Merge pull request #6 from dpla/7657-fix-sitemap-generation

Use UTC timestamps everywhere/improve sitemap generation (fixes #7657)