Introducing storage interface for CDN Stored files

[mapkyca]

Here's what I fixed or added:

Added an interface to allow for CDN storable files

Here's why I did it:

To support direct URL for a file stored on a CDN (for use with IPFS etc)

Refs #2322

Checklist:

• This pull request addresses a single issue
• If this code includes interface changes, I've included screenshots in this Pull Request thread
• I've adhered to Known's style guide
• My git branch is named in a descriptive way - i.e., yourname-summary-of-issue
• I've tested my code in-browser
• My code contains descriptive comments
• I've added tests where applicable