Fix issues in SitemapService

[matsbov]

This PR fixes the following issues in SitemapService (https://github.com/AtlasOfLivingAustralia/collectory/issues/244):

• The sitemap index file creates <url> tags that should be <loc>
• id is used instead of uid when creating url:s which is incorrect
• Scheduled (daily) generation isn't enabled unless the sitemap is generated manually from the admin UI first
• Scheduled runs crash with an exception (manual runs work fine)
• Repeated runs of sitemap generation will create sitemap files that don't match the sitemap index file

See code comments below for details.