Improve Timeline Image Loading

[jkennedyvz]

Images are retrieved by the ashirt backend and then passed to the client, and timelines are loaded at once as we don't have pagination in place. This causes very slow loading times for timlines that contain more than a few hundred pieces of evidence.

Allowing clients to retrieve evidence out of band could reduce timeline loading times without relying on pagination.

When evidence is on s3 or gstorage we can generate pre-signed URLs to give back to the client. I'm not sure how we would authenticate requests for hosting static files, or if we should modify that at all.

[jkennedyvz]

Closed with #956