optimize png images losslessly using zopflipng

app/assets/images/auth_buttons/azure_64.png

app/assets/images/auth_buttons/bitbucket_64.png

app/assets/images/auth_buttons/facebook_64.png

app/assets/images/auth_buttons/github_64.png

app/assets/images/auth_buttons/gitlab_64.png

app/assets/images/auth_buttons/google_64.png

app/assets/images/auth_buttons/twitter_64.png

app/assets/images/bg_fallback.png

app/assets/images/dark-scheme-preview.png

app/assets/images/emoji.png

app/assets/images/emoji@2x.png

app/assets/images/gitlab_logo.png

app/assets/images/gitorious-logo-black.png

app/assets/images/gitorious-logo-blue.png

app/assets/images/icon-link.png

app/assets/images/images.png

app/assets/images/monokai-scheme-preview.png

app/assets/images/msapplication-tile.png

app/assets/images/no_avatar.png

app/assets/images/no_group_avatar.png