Move all dialog buttons to the right and fix their order

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.

res/css/_common.scss

@@ -222,6 +222,11 @@ textarea {
     word-wrap: break-word;
 }
 
+.mx_Dialog_buttons {
+    padding-right: 58px;
+    text-align: right;
+}
+
 .mx_Dialog button, .mx_Dialog input[type="submit"] {
     @mixin mx_DialogButton;
     margin-left: 0px;