Optimize PNG images losslessly via Google zopflipng

[PeterDaveHello]

• Commit message has a short title
• Commits are squashed
• The build will pass (run ember test from the repo root - will be core/client if working from the submodule in Ghost).

This change re-compressed 15 PNG images losslessly, to make the client a little bit thinner and loading faster ⚡️

 public/assets/img/404-ghost.png              | Bin 3681 -> 3003 bytes
 public/assets/img/404-ghost@2x.png           | Bin 7958 -> 5529 bytes
 public/assets/img/ampicon.png                | Bin 8715 -> 3056 bytes
 public/assets/img/ghost-logo.png             | Bin 8638 -> 6516 bytes
 public/assets/img/install-welcome.png        | Bin 240204 -> 238626 bytes
 public/assets/img/large.png                  | Bin 1912 -> 1678 bytes
 public/assets/img/medium.png                 | Bin 400 -> 126 bytes
 public/assets/img/small.png                  | Bin 426 -> 409 bytes
 public/assets/img/touch-icon-ipad.png        | Bin 494 -> 470 bytes
 public/assets/img/touch-icon-iphone.png      | Bin 640 -> 622 bytes
 public/assets/img/unsplash-404.png           | Bin 2196 -> 2170 bytes
 public/assets/img/unsplashicon.png           | Bin 2786 -> 2459 bytes
 public/assets/img/user-cover.png             | Bin 23563 -> 15205 bytes
 public/assets/img/user-image.png             | Bin 2421 -> 1478 bytes
 public/assets/img/users.png                  | Bin 49253 -> 46993 bytes

[coveralls]

Coverage remained the same at 71.787% when pulling 76d17b1 on PeterDaveHelloKitchen:optimize-png-image into 63fe1b1 on TryGhost:master.