Add naive sitemap generation for the cover art tab.
Support adding extra columns to the SQL for getting IDs for a batch, and custom functions to determine priority values.
Reorganize SQL generation and split out building of suffix info. Include suffix in the suffix info initially.
Merge branch 'sitemaps-coverart' into beta

* sitemaps-coverart:
  Reorganize SQL generation and split out building of suffix info. Include suffix in the suffix info initially.
  Support adding extra columns to the SQL for getting IDs for a batch, and custom functions to determine priority values.
  Add naive sitemap generation for the cover art tab.