Fix race condition in deep linking

[alexgb]

If a user returns to the app before the oauth login flow completes, a hanging deep link event handler will cause the next oauth flow to trigger two token requests. In the case of google oauth, this will result in an invalid_grant error.

Steps to reproduce:

1. Configure an app to oauth using google
2. On iOS, trigger an oauth flow and return to the app using the "back-to" button in the status bar
3. Trigger oauth again and this time complete the flow and click the open button in the dialog to deep link back to your app
4. You should now see an invalid_grant error

This PR fixes this case by ensuring that only a single active deep linking url handler can exist at a time.

[adamjmcgrath]

Hi, sorry am away at the moment so not been able to look at this. Back late next week so will take a look then.

[adamjmcgrath]

lgtm, apologies for the wait...