Implementation of ios push notifications

[kunall17]

• Currently untested, due to some issues of apns certificate on chat.zulip.org (Will be resolved soon)
• Not sure if Bundle ID should be the same as the certificate ID, but a provisioning team needs to be create to test this on a device
  
• The displaying content part of the notification is mostly related from the server (unlike GCM), hence a PR will be landing there.

[smarx]

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[borisyankov]

At a first glance the code looks ok. I'll do a more thorough review and merge if OK.
Make sure the code does not crash the app even if the server has no support for notifications, both for iOS and Android, because we might have to connect to older servers that don't have that.

[zulipbot]

Heads up @kunall17, we just merged some commits that conflict with the changes your made in this pull request! You can review this repository's recent commits to see where the conflicts occur. Please rebase your feature branch against the upstream/master branch and resolve your pull request's merge conflicts accordingly.

[borisyankov]

Rebase and try to test and improve the code and prepare it for the server with a certificate. Likely we will have it very soon and then it will be highest priority to make it work.

[borisyankov]

👍

[borisyankov]

Make sure tests pass.

[kunall17]

Done

[borisyankov]

Nice. Getting it in.