Keep access token between sessions

New access tokens are now created on every reload of Embedded Genesys. While this works just fine, it's less then ideal. Token should be persisted between user's sessions.

Access token should be stored in localStorage when it is available, otherwise fall back to a cookie.

Update checkAccessToken in genesys.tsx to use localStorage or cookie to load/save the token.

If token is not available or has expired, fetch and save the new token.