Batch notifications during active conversations

#### Describe the bug
My Android app makes my phone buzz off the desk when there is an active conversation in a room.

#### To Reproduce
Steps to reproduce the behavior:
1. Lock phone
2. Have an active conversation happen in a room with noisy notifications.
3. Phone buzzes non-stop, draining battery a lot faster than it should (and it's very disruptive)

#### Expected behavior
If a user messages me or if there are multiple messages in quick succession in a room, the notifications should be batched and only the first one should be noisy. The notifications which come in quick succession after the first one should not trigger more noisy notifications and some time should pass before the next noisy notification from that chat.

Maybe notify for the first message and then not notify for the following 30 seconds if the messages are less than a second apart?

~~I believe Telegram batches notifications like this.~~ Telegram notifications are broken in different and exciting ways.

See also vector-im/element-meta#360

#### Smartphone (please complete the following information):
- Device: Pixel 2
- OS: Android 11

#### Additional context
- App store, version 1.1.15 [40101152] (G-b2969)
- Homeserver: matrix.org