Push and notification improvements

This is a meta issue to quickly list things to solve. Corresponding issues will be created:

# Push technical issues
- [x] e2ee keys lost in the push extension. The app never got them
- [x] Out of memory errors in push extension
- [ ] Request `com.apple.developer.pushkit.unrestricted-voip` to [Apple](https://developer.apple.com/forums/thread/663956)
- [ ] Or consider this other entitlement, [com.apple.developer.usernotifications.filtering](https://developer.apple.com/documentation/bundleresources/entitlements/com_apple_developer_usernotifications_filtering), to hide unexpected notifications
- [ ] Use /event again and throttle /sync requests if we can

# Notification UX issues
- [x] #4659
- [x] #4653
- [x] Have dedicated notifications for reactions
- [x] Notice events
- [x] Have dedicated notifications for errors: "Sorry, we failed to fetch the message content, open the app", etc
- [x] #4660
- [ ] Have dedicated notifications for media
- [ ] Consider appropriate content (or silence) notifications from mention-only rooms when not mentioned ([discussion](https://github.com/vector-im/element-ios/pull/4662#discussion_r691077847)).
- [ ] Dismiss "Notification" notifications when another comes in to avoid stacking.
- [ ] ~~Have notification for invite (not sure if it works)~~
- [ ] #4710 (pending Xcode 13 support in the project and on CI).

Later:
- [x] Hide "Message" or improve notifications for technical events. Will be fixed with `unrestricted-voip`
- [ ] Have more actions (e.g. accept/decline room invites)