Capture and store last seen user flags from the booru

Main use case for these flags is to notify when something they're trying
to use will not be usable due to authorization or their role on the
site. Specifically, it will be also used later to detect when user
is a part of staff and show them staff-specific features.

manifest.json

@@ -28,7 +28,8 @@
                 "*://*.furbooru.org/*"
             ],
             "js": [
-                "src/content/deps/amd.ts"
+                "src/content/deps/amd.ts",
+                "src/content/user-details.ts"
             ]
         },
         {