[iOS] Create empty notification service extension
Summary: Add an empty extension using XCode new target template. The service extension is executed when the app is backgrounded or killed. When the app is running, this code is not called.
Test Plan:
Modify prepareIOSNotification in send.js by setting notification.mutableContent = true. Modify the code in service so that the body gets modified.
Test on a physical device:
- Run the app and send a message from other client - the notification should be displayed with the original body.
- Background the app and send a message - the modified notification should be displayed.
- Kill the app and send a message - the modified notification should be displayed.
Reviewers: karol-bisztyga, atul, jimpo, ashoat
Reviewed By: ashoat
Subscribers: benschac, ashoat, Adrian
Differential Revision: https://phabricator.ashoat.com/D3162