Delete subscription instead of creating deactivation when page isn't reloaded

https://trello.com/c/l2vLI2yj/353-address-repos-with-subscriptions-that-weren-t-enabled

There is an issue where a subscription can be created for a repo but the subscription cannot be deleted if the page is not reloaded.

This addresses the issue by updating the JavaScript objects that represent the repo with the newly created subscription.
Thoughts on one-linning both `catch` and `finally` (if they fit)?