1. Fix some bugs in local_storage
2. Refactor unit tests to not set Media.key explicitly (unless it's well-known beforehand, which it isn't)
3. Limit length of URL for 'url' type path_generator
4. Throw an error if 'save_to' of local storage is too long
5. A few other tidyups
Context for a specific url/item is now passed around via the metadata (metadata.set_context('key', 'val') and metadata.get_context('key', default='something')
The only other thing that was passed around in ArchivingContext was the storage info, which is already accessible now via self.config
