Implement push notification

Summary: upon silent/data push, fetch transaction history in the background and if we can find the transaction (why not?), show the user a notification

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* AlphaWallet apps calls and the server endpoint to register a wallet 0xb0b
* Server monitors 0xb0b for change of receive of native crypto, ERC-20, ERC-721, ERC-1155
* Server sends a silent push to all AlphaWallet apps device tokens that registered for 0xb0b
* Upon receive a silent push for 0xb0b, AlphaWallet apps fetches/refreshes transaction history for 0xb0b in the background
* AlphaWallet displays a notification that "0xb0b has received X ETH" or "0xb0b has received Some App 123"
* User tap the notification to see that transaction/activity

1. Verify we get notifications for receiving of native crypto, ERC-20, ERC-721, ERC-1155 (did I miss anything?) on Ethereum mainnet.
2. iOS test getting pushes too like Android did
3. We build it in AlphaWallet and try it for a few hardcoded wallets that we own. Ship it
4. Extend it to more people or everyone (depending on confidence level and how much work to rollout) and ship it.

Makes sense? Did I miss anything? So I think you can proceed with (1) and (3) and on iOS side, we'll do (1) and (2).

Just to be safe, remember gate it with a feature flag in case it isn't ready.