Downloading static video to offline storage via Background Fetch API

[jwold]

Feature description

As a user, I should be able to download a static video to offline storage via background fetch API

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Acceptance criteria

Design note and AC: Presentation of file being downloaded is different between background fetch and fetch API. On background fetch there is a banner on system that says being downloaded, will happen regardless of leaving browser. Fetch API handled by application, need indicator built into application to expose progress to user.

They also should be aware of where their downloads are going.

Relates to #24, #25, #26, #35

Implementation brief

QA testing instructions

Demo

Changelog entry

[jwold]

@derekherman can you validate if this should still be in scope for Phase 1? It sounds like it may not be. cc @dero