Support multiple accounts

I've fixed the patch for the latest 2.x-dev.
I haven't fully tested though.
I would improve logic for the settings form. It's looks weird if we have only one store and there are a couple of the errors on first visit on the form after applying the patch. Maybe we can check some checkbox to enable per site support if there are multiple stores on a site?
And I'm not sure why we need to generate API keys and use them there as the module actually doesn't make any requests to the ShipStation API (or I've not found if it does). We use custom username and password for the module's endpoint to verify requests from the ShipStation.

I've created new branch and MR with some improvements in UI - admin form for the module.
I've added a checkbox 'Use per store settings' and when it is checked - hide the user name and password fields and show grouped fields per each store. The checkbox is displayed when there are multiple stores on a site. We can add also other settings per each store if needed.

Roman this looks like a good start, I can see one blocker on the pr. If "Use per store settings" is checked its not clear to the user that they don't have to enter settings per each store. For example:

User has:
EU SS account
UK SS account

On the site they have these stores:
EU Store
UK Store
US Store

US store and UK store both use the UK SS account and the EU Store uses the EU SS Account. Does the user have the enter the UK SS account details twice?

Final comment could we change "Use per store settings" label to "Shipstation Account per store" or something similar?

Roman just tried applying your MR against the latest version and it does not apply

c_archer → closed merge request !8

Created a new MR for this which simplifies the logic.