Push notification improvements

Given we can only show 1 notification at a time, we should provide a summary of all rooms we have received notifications from so far. So we get this:

## 1 message in a single room (implemented)
**Alice**
Hello

## 2 messages in a single room (implemented)
**Alice**
New messages

## Multiple messages in up to 5 rooms (not implemented)
**New messages**
2 in Public Room
1 in Alice

## Multiple messages in more than 5 (or less?) rooms (not implemented)
**New messages**
5 in Public Room
2 in Alice
1 in Bob
1 in Charly
1 in Hydrogen
and 3 more rooms

We can use the notification data to store the information from the push message to aggregate the appearance of several messages into one notification.

Start by tracing the push-notification handling and the notification data mentioned in the issue. Implement aggregation for messages from multiple rooms, including room counts and an overflow summary beyond the chosen room limit; done means one notification accurately summarizes all received rooms.

Written by the indexing model from the issue text.