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

* Sitemaps require `<lastmod>` dates to be in W3C Datetime format
* Use `ingestDate` from items to populate `<lastmod>`
* Move configuration loading logic outside functions
* Add configuration option for local sitemap path -- required change to `akara.ini`
Is this assertion necessary? Looks tautological in light of the previous one.