Minor: use STATIC_DIR const for static middleware too.
Config: add ALLOW_NEW_CONTENT to support closing our doors.

Fixes #43.
Minor: standardize Content class on 4-space indent.
Data: remove Content `self` & `type` properties.

Just for v1 exact compatibility. We can add them back in v2.
View: don’t hardcode content DZI path.
Fetcher: properly catch & propagate download errors.