Add FreshRSS widget (#1377)

* feat: add FreshRSS widget

* refactor: revert credentialed.js

* refactor: custom proxy handler for FreshRSS

* refactor: cache the token as long as possible

During installation, the salt is generated and remains constant unless
the user re-installs the FreshRSS instance.

src/widgets/freshrss/widget.js

@@ -0,0 +1,13 @@
+import freshrssProxyHandler from "./proxy";
+
+const widget = {
+  api: "{url}/api/greader.php/{endpoint}?output=json",
+  proxyHandler: freshrssProxyHandler,
+  mappings: {
+    info: {
+      endpoint: "/"
+    }
+  }
+};
+
+export default widget;