The _admin_tinymce_overrides_js.tpl initializes TinyMCE and adds svg elements as valid to the editor. Copy the file _admin_tinymce_overrides_js.tpl, or merge with existing, to your Zotonic site templates directory.
To enable svg sanitization in Zotonic you also have to add {html_elt_extra, [ <<"svg">> ]}, to you sites config file. (Without trailing "," if it is the last one.)
After this you should be able to paste and save svg elements into the editor when in source mode.