fix: configure fetch to send cookies

Previously, when XMLHttpRequest was used, cookies ware sent
when getting /-/verdaccio/logo & /-/verdaccio/packages

After switching to fetch in sha fef7ee7
cookies are no longer sent.

This commit configures fetch in the webui to send cookies to the same
domain.

> The "same-origin" value makes fetch behave similarly to
> XMLHttpRequest with regards to cookies.
> https://github.com/github/fetch#sending-cookies

src/webui/utils/api.js

@@ -26,6 +26,7 @@ class API {
 
       return fetch(url, {
         method,
+        credentials: 'same-origin',
         ...options
       }).then(handleErrors);
     }