Allow Users to Specify Idle Logout Timeout

This branch implements allowing users to specify the number of minutes of idle time before being automatically logged out. The specified value is saved to local storage and the local storage key is defined as an exported option in the configuration files for each environment.

The should resolve issue https://github.com/stellar/stellar-client/issues/583.

As an aside, there's a bit of a code smell in that the session service is responsible for managing these values, but Wallet needs them in order to set the expire value for ipCookie. Having Wallet depend on session would, of course, create a circular dependency, so no immediately obvious fix presented itself.