Create icons for link/image pickers

CKEditor uses a couple of png files as the icons for the pickers buttons.

TinyMCE seems to only support svg files and requires a somewhat more elaborate setup.: currently we use the method addIcon, to add an inline svg in our hippopicker: (/community/cms/richtext/tinymce/plugins/src/main/resources/tinymce/plugins/hippopicker/plugin.min.js).

This isn't ideal, and we can consider creating a custom icon pack:

https://www.tiny.cloud/docs/tinymce/latest/editor-icons/#icons_url and  https://www.tiny.cloud/docs/tinymce/latest/creating-an-icon-pack/