Automatically set or unset a user's preferred "billing account" when they join or leave a single team with usage-based billing enabled

[jankeromnes]

When a user has not selected a specific "billing account" or "re-attribution ID" yet, we can set it automatically in the following cases:

• The user joins a team with usage-based billing enabled
• One of the user's teams upgrades to usage-based billing

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Similarly, when a user has selected a specific team as "billing account" or "re-attribution ID", we should probably unset this value again when:

• The user leaves, or is removed from, the selected team