Use Matrix SDK to store auth tokens in local storage

[pmaier1]

Switch from in-memory storage to local storage to improve user experience.

https://github.com/matrix-org/matrix-js-sdk/blob/3a3be36f4c274da2c8233271605f43adeae226e5/src/store/indexeddb-store-worker.js

http://matrix-org.github.io/matrix-js-sdk/1.0.1/module-store_indexeddb-IndexedDBStore.html