R1:25bd14a9b1e4

Respect "End Date" for recurring Calendar events in detail UI

Summary:
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.

Test Plan:
- Visited…