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.
