Fixes T11396. Currently, you can keep clicking "Next >" forever to generate infinite instances of an event, even if it has a set end date.

Likewise, you can visit /E123/999999 or whatever to stub out the 999999th instance of an event.

Instead:

• Before creating a new stub, make sure it happens before any end date.
• 404 stubs if we can't create them.
• Disable the "Next >" button if it isn't valid.

• Visited /E123/9999 for an event with a recurrence end date, got 404.
• Clicked "Next >" on an event with an end date, got new events until I hit the end date.