~emersion/goguma#138: iOS feature parity

Set DarwinInitializationSettings and DarwinNotificationDetails for flutter_local_notifications (possibly more)
~~Turn on app_links to handle irc:// URLs~~
~~Turn on share_handler~~
~~Add support for APNS for push notifications~~
- ~~Add APNS support to pushgarden~~
- ~~Wire up flutter_apns~~
Add iOS support to workmanager to poll for new messages in the background
Add iOS support to flutter_background for IRC servers missing chathistory support

(In general, grep for isAndroid and see what's missing.)