Previously background contexts where created where necessary (e.g. in
GCS interactions). Should I begin to use request timeouts or other
context-dependent things in the future, it's useful to have the actual
HTTP request context around.
This threads the request context through the application to all places
that need it.
The point at which files are moved happens to also (initially) be the
point where the `layers` directory is created. For this reason
renaming must ensure that all path components exist, which this commit
takes care of.
The filesystem storage backend can be enabled by setting
`NIXERY_STORAGE_BACKEND` to `filesystem` and `STORAGE_PATH` to a disk
location from which Nixery can serve files.
This abstracts over the functionality of Google Cloud Storage and
other potential underlying storage backends to make it possible to
replace these in Nixery.
The GCS backend is not yet reimplemented.
