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.

public/locales/en/common.json

@@ -98,6 +98,10 @@
         "leech": "Leech",
         "seed": "Seed"
     },
+    "freshrss": {
+        "subscriptions": "Subscriptions",
+        "unread": "Unread"
+    },
     "changedetectionio": {
         "totalObserved": "Total Observed",
         "diffsDetected": "Diffs Detected"