Enhancement: add WEB_OPTION_TOKEN_STORAGE_LOCAL env variable

We introduced the feature to store the access token in the local storage, this feature is disabled by default,
but can be enabled by setting the env `WEB_OPTION_TOKEN_STORAGE_LOCAL` to `true`.

https://github.com/owncloud/ocis/pull/6760
https://github.com/owncloud/ocis/pull/6771
