Add feathericons for viewer buttons

app/javascript/packs/controllers/viewer_controller.js

...openseadragon/images/feathericons/fullpage_grouphover.png

public/openseadragon/images/feathericons/fullpage_hover.png

...ic/openseadragon/images/feathericons/fullpage_pressed.png

public/openseadragon/images/feathericons/fullpage_rest.png

public/openseadragon/images/feathericons/home_grouphover.png

public/openseadragon/images/feathericons/home_hover.png

public/openseadragon/images/feathericons/home_pressed.png

public/openseadragon/images/feathericons/home_rest.png

public/openseadragon/images/feathericons/info_grouphover.png

public/openseadragon/images/feathericons/info_hover.png

public/openseadragon/images/feathericons/info_pressed.png

public/openseadragon/images/feathericons/info_rest.png

public/openseadragon/images/feathericons/next_grouphover.png

public/openseadragon/images/feathericons/next_hover.png

public/openseadragon/images/feathericons/next_pressed.png

public/openseadragon/images/feathericons/next_rest.png

...openseadragon/images/feathericons/previous_grouphover.png

public/openseadragon/images/feathericons/previous_hover.png

...ic/openseadragon/images/feathericons/previous_pressed.png