Require users to contact sales to upgrade to Enterprise team

[knolleary]

Part of https://github.com/FlowFuse/customer/issues/233

• Add option to TeamType object to flag it as requiring admin permission to upgrade to
• Type should still be listed as an option on the Upgrade Type page; but, when viewed by a non-admin user, should show a 'contact us' button when selected.
• Clicking the button should open a new tab on the Contact Us page.
• The API should reject upgrade requests from non-admin users.

Need to decide how much of this is configurable; eg the url the contact us button takes the user to, the text 'contact us' itself. We have successfully avoided hardcoding FFC specific choices into the billing code - and made things like this configurable.

The workflow would then be:

1. User creates regular team - contacts us as they want enterprise
2. Discussions had, agreement reached.
3. Invoice is manually created and sent out
4. Admin uses the 'unmanaged billing' option to upgrade the team to enterprise and disconnect from managed billing.

[knolleary]

An alternative option would be for the contact us button to be configured to trigger a HS workflow that automatically notifies Sales. Need to understand the mechanics of that; presumably the button would need to trigger a platform API that in turn pings hubspot (rather than doing it directly in the page... not sure).

[knolleary]

The implementation in #4343 requires the teamtype to be configured with the id of a hs form that should be submitted automatically when the 'contact' button is clicked.