added new icons and updated header to use them

cc @jebeck i also updated the top bar to use the icons as blip does
I don't think these should be props. Just build the `'icon-back'` etc. into the `backClass` and `nextClass` and change the HTML to `<i>` tags instead of `<img>`. This is necessary to keep the `inTransition` and `atMostRecent` interactions (greying out arrows when they shouldn't be clickable) as well. Make sense?