Add unlimitedStorage permission (#930)

IndexedDB storage may be evicted by the browser at its discretion [1].
Granting the unlimitedStorage permission ensures that IndexedDB data
will not be evicted [2], and allows the extension to store more than
5MB of data if needed [3].

[1]: https://web.dev/storage-for-the-web/#eviction
[2]: https://crbug.com/680392#c5
[3]: https://developer.chrome.com/apps/declare_permissions#unlimitedStorage

manifest.json

@@ -18,6 +18,7 @@
     "webRequestBlocking",
     "contextMenus",
     "storage",
+    "unlimitedStorage",
     "alarms",
     "identity",
     "<all_urls>"