Fixes https://github.com/vector-im/riot-web/issues/5689
Some dialogs had their own CSS that prevented the buttons from being placed in the right spot. This has been fixed by using a generic standard for the buttons. The only strange dialog that needed more CSS was the devtools dialog due to the header.
Not all dialogs have been checked - I spot-checked about half of them and verified the CSS manually on an established account. It's hard to get at all the dialogs without convoluted testing.
Continues from Matthew's work: adds a feature flag & panel in
user settings to create a backup.
Can't restore a backup yet, nor even continue backing up to the same
backup after a refresh.