Add interface for setting a global notififcation sound

[Half-Shot]

Continuing the discussion from #2105, Element has an interface to set a custom sound per room, but not yet one per room. Global custom sounds already work, but are just not exposed.

For users looking to change this and are happy to fiddle around in devtools, these instructions will work.

What would you like to do?

The ability to set a custom notification sound globally across the client from the settings menu.

Why would you like to do it?

We already have the functionality, it's just not exposed.

How would you like to achieve it?

Well, a settings menu..

[Ryonez]

Thanks you for those instructions. The default sound is rather loud and harsh to me, it's good to be able to change it.

[amunrarara]

I support this feature request. The default sound has caused me some horrible jump-scares, more than once, especially when my headphones are on. It should be an easy fix, putting in this option, and would increase product quality tremendously.

[kenwuuu]

In progress. Follow that issue instead

[eslerm]

Hi @robintown o/

From your comment in @kenwuuu's PR:

The first order of business will be to get our product team's input on whether this is in fact the kind of change that we want to make to our notification settings.

Could you please raise that this issue is a frequent accessibility problem to the product team? This issue is also discussed in #5891. It feels like low-hanging fruit for improving the user experience and accessibility of element.