Generate lifestream.css on build

This was a pet peeve of mine actually. Basically this enables us to base64 encode the images upon build, removing a few contribution steps and making it easier to add a service (I know, not our biggest concern). I eventually did it because I saw the mendeley icon was missing for some reason.

In the future it might be nice to also optimize the images before encoding, though I'll have to research this a bit.