Add uploaded_blob context manager, allow calling of model clean() with overridden settings

Contents (#84)

⚠️ This release has been yanked from pypi as it introduces a mismatched signature when uploading blobfields from the django admin. Use 0.18.1 which patches the problem.

New features

• Add uploaded_blob context manager, useful for unit testing

Fixes

• Allow field cleaning to happen inside an overridden context
• Correct implementation of headers for limiting content length range
• Make upload_blob woth with default destination path helper

Operations

• Remove deprecated setting from devcontainer json

Refactoring

• Remove redundant context in test

Testing

• Test uploaded_blob context manager