Remove branded images and their usage

app/javascript/images/elephant-fren.png

app/javascript/images/elephant_ui_disappointed.svg

app/javascript/images/elephant_ui_greeting.svg

app/javascript/images/elephant_ui_plane.svg

app/javascript/images/elephant_ui_working.svg

app/javascript/images/logo.svg

app/javascript/images/logo_alt.svg

app/javascript/images/logo_full.svg

app/javascript/images/logo_transparent.svg

app/javascript/images/preview.jpg

app/javascript/mastodon/features/compose/index.js

app/javascript/styles/mastodon/components.scss

app/javascript/styles/mastodon/modal.scss