Image Optimization - prevent future memory limits

[mhagel]

• Implement a fallback image on the Impact cards (or just use unoptimized images)
• Test and upgrade heroku-redis to a higher plan. I tried to do this, but it broke things - needs more attention.
• Make server code less brittle. (When I tried to upgrade, registry-server couldn't find the new REDIS_URL variable. This caused the code to fail, and the whole server was down.

[wgwz]

@mhagel marking this as closed via https://github.com/regen-network/registry-server/pull/241