* ios: get device token for push notifications
* ios: receive messages when background notification is received
* add notifications API, update simplexmq
* chat API to register and verify notification token
* update AppDelegate to recognize different notification types, update simplexmq
* core: api to enable periodic background notifications
* update simplexmq
* chat API to delete device notification token
* use base64url encoding in verification code
* update simplexmq for notifications
* receiving messages in the background and sending local notifications
* show notifications in foreground and background
* presentation logic for notification options when app is in the foreground
* background refresh works
* remove async dispatch
