Split sitemap when it is getting too big

To ensure we will get the right `content`
directory.
Fix #629

adding back default in rebuild

Fix #620

Closes #607

Fix #604

Closes #582

Closes #576

Closes #521

Closes #533

Aliases that have no directory nesting sneakily avoid the code path that ensures the directories exist

Local CSV/TOML/JSON loading Tera function

Close #446
Close #260
Close #478
Close #284
Close #480

Switch to an `Option<usize>` for the serialized value of `rss_items`.
This lets us just set a blank value in the configuration and thereby
include *all* items.

This is a backwards-compatible change; it does not affect the behavior
of existing configurations.

Fixes #468. Closes #471.

Fix #122

Fix #205

This allow to use base_url in robots.txt, to reference a sitemap for
example.

Closes #443