Add the possibility to have per account billing instead of per workspace billing

We are investigating adding an Enterprise pricing tier for Bitbucket similar to that offered for Jira and Confluence. The primary benefit of the Enterprise license is billing per user across multiple product instances.  https://support.atlassian.com/subscriptions-and-billing/docs/manage-your-bill-for-enterprise-plans/ We are also investigating better ways to allow customers to consolidate all projects and repositories into a common workspace to obviate the need for separately billed workspaces.