No need to check for postsale or cron script

We previously did that to prevent creation of a cart by default,
but this will now only happen on call to Isotope::getCart()
CS
Added store_id configuration to root page
Retrieve active store_id from Cart instead of Config object
Use setConfig instead of overrideConfig

If the value would be null, the next call to getConfig() would initialize
the default anyways
Find the last highest order ID by store_id instead of config group
Improved label
Cart is initialized for the current root page
Deleted resetConfig, no longer necessary
Deleted store_id from config
Merge branch 'feature/store_id' into develop