Enhancement: Allow local storage for auth token

We've introduced a new env var WEB_OPTION_TOKEN_STORAGE_LOCAL, when set to true(default), the auth token will be stored in the
browser's local storage instead the session storage, this will effect in a persisted login state across multiple
browser tabs.

https://github.com/owncloud/web/pull/9386
https://github.com/owncloud/web/issues/9325
