Improve UI/UX regarding Key Backup

Hello,

This is related to https://github.com/vector-im/riot-web/issues/14416. Thank your for your work on Riot!

First this toast:

What is the relation between secure message keys and key backup? What this toast is really about? What keys exactly are backed up? Why am I apparently prompted to do something?

Clicking on the toast leads to this screen:

The text and the green check mark seem to indicate that everything is fine. So what was the purpose of this toast? Do I need to do something? And what will happen if I click on those 2 buttons?

Clicking on the "Restore from backup" shows a screen like that:

OK but what does it really mean from the user's perspective?

Then it leads back to the previous screen. The check mark is still green and the "Restore from backup" button is still clickable, so it seems like clicking on the button didn't have any visible effect.

If I am not mistaken, all of this is related to conversation history, right? Then the toast, the setting screen and the buttons should convey why it's interesting from the user's perspective to review these settings, and what is the advantage of clicking on the buttons (and what is at risk when clicking on "Delete backup").