Some SCMs do not have avatar URLs that support resizing. Provide an end-point that will cache and resize external avatars on behalf of an AvatarMetadataAction

Acceptance criteria:

• Cache should be size bounded to prevent excessive memory usage. Where remote images are large, they should be scaled down before being cached in order to reduce the memory footprint
• Concurrent requests to fetch images from remote URLs should be capped
• When a remote image is unavailable, a consistent placeholder avatar should be generated.
• Resized avatar images should be served with suitable cache-control headers to ensure that images are retained in co-operating client browser caches for up to 24h