Usage chart doesn't include additional reporting subscriptions

[mbarton]

Applies to the mbarton/groups branch

Customers looking at their usage will only see a graph for their current subscription. If they have additional reporting subscriptions (for example new key or usage from an old apim instance) it will not be included.

At the moment we don't store which product the additional reporting subscriptions apply to so it's not possible to do this.

Ideas:

• Store product with the subscription (needs a more complicated Discourse UI though)
• Look up each subscription in the Azure APIM API to get the product before returning usage

NB this doesn't affect the overall usage report as that has separate rows per subscription (see #1)