Security settings: Replace the "Synchronise" button

[manuroe]

that nobody understands.

Replace it with what we had before, ie the "Restore from Backup" button. This means we have to resurrect the cross-signing section. This screen will then offer same services as the same same screen on android and web.

FTR, the goal of the "Synchronise" button was to manage both cross-signing and key backup.

[manuroe]

Current state:

[manuroe]

The same services as element-web will be proposed. The "Advanced" detailed information will be managed later.

and