Remember signed in Harvest user across multiple tickets

Only attempt to pre-populate the store from app settings where there is no key already present in the store for that setting.

This is so that a ZD user can sign in to the Harvest app in one ticket tab, then open another tab and expect the Harvest app in the new tab to auto log in for the same Harvest account.

/cc @jamesarosen @jakeisonline @jwswj