Provide a "single-subscription" mode

Right now the module is set up in a way that it's unlikely that a user would create multiple subscriptions (of different plans of course). But it's still possible for a user to do so by manually going to the user/x/subscription/signup page and selecting a second (or third) plan. I accidentally did this once and ended up with two different plans, which would have been confusing for users of my site which can only have one plan at a time.

Additionally, the current split between the "Change plan" and "Cancel" links being part of the subscription links while "Invoices" and "Update Billing Information" is a sub-tab is confusing when there is only a single subscription on the site. Users would expect that all these options would all be in one place or the other (preferably as local tasks in my opinion) if there is only a single subscription allowed.

If only a single subscription is allowed, visiting the user/x/subscription/signup page should direct you to the change plan page instead.

And finally, if this "Single-subscription" mode is not enabled, then the "Signup" page should be present at all times, probably with the name "Add plan" instead of "Signup", since the user can continuously add more plans as desired.

Recurly still has the limitation that a user cannot sign up for the same subscription more than once, so any plans that the user already has subscribed to should not be shown when adding additional plans.