No Portal access for customers in reseller deals

Problem

In #1169, we've identified new workflows necessary to better facilitate reseller deals. However, those new workflows, like allowing resellers access to the Customers Portal, could have big implications for the app and implementing those features could take significant effort.

Proposal

As an MVC, we want to block all users from reseller deals from gaining access to the Customers Portal. The Customers Portal should still create Customer records, necessary for keep existing features working like subscription creation and license management, but simply do not allow those Customers to sign in. In addition, these Customers should not receive welcome emails from the Portal. They should however, still receive emails for EULA acceptance and license delivery.

Result

The end user in reseller deals can't login to the Customers Portal for reseller deals.

/cc @amandarueda