Element X: hide notification contents

### Your use case

#### What would you like to do?

Hide the content of the notifications displayed on both Android and iOS devices.

#### Why would you like to do it?

While both OS let you hide the notification contents in the lock screen, there is still the issue of people spying on any notification displayed while you're actively using the device, or the OS - allegedly- being able to spy on the contents of the notifications (technically, if it can spy on the notifications it can spy on *any* text that's drawn on screen but...).

#### How would you like to achieve it?

By having a user the setting can toggle to hide the notification contents at will. This would either hide both the sender and the notification content or just the content, replacing it with some generic string like '(Someone/Username) sent a message' or 'You received a message'.

This may be a bit trickier than just doing that because - at least on Android - we have a notification summary for any room, which means receiving several notifications and expanding the summary would read like this to the user:

```
A room:
-----------------------
Alice sent a message.
Bob sent a message.
Alice sent a message.
Alice sent a message.
Bob sent a message.
Bob sent a message.
```

Or if we hide the sender's display name too:

```
A room:
-----------------------
Someone sent a message.
Someone sent a message.
Someone sent a message.
Someone sent a message.
Someone sent a message.
Someone sent a message.
```

Which is way worse in my opinion. In that case, maybe we should keep at most 1 item in the summary, but this complicates the logic quite a bit.

### Have you considered any alternatives?

The only alternatives would be:

- The user to disable pop-up notifications on Android which would still display an icon in the status bar and vibrate and notify with sound. - not sure if this is possible on iOS.
- Disable notifications altogether.
- Keeping it as is, hoping Android or iOS implement this feature in the future.

### Additional context

Original request on Element X Android: https://github.com/element-hq/element-x-android/issues/3231

Start with the original Element X Android request linked in this issue, then identify the corresponding notification handling work for iOS. Define how the user setting hides sender and message contents on each platform, including notification summaries, and verify that the resulting notifications expose no message text.

Written by the indexing model from the issue text.