Make ImageDownloader mockable

[plflanagan]

• I've read, understood, and done my best to follow the CONTRIBUTING guidelines.

This pull request includes (pick all that apply):

• Bugfixes
• New features
• Breaking changes
• Documentation updates
• Unit tests
• Other

Summary

In order to write downloading tests without needing access to the internet, it is necessary to be able to mock an ImageDownloader.

Implementation

This PR makes a protocol ImageDownloading that can then mock an ImageDownloader. This requires making the Response struct to have a public init as well.

Test Plan

No further testing added.