Add independent set up / reset actions in Settings

This adds set up and reset actions to each of cross-signing and secure backup
that do separate things, rather than mixing concerns together. (It's temporarily
still a bit of lie for backup, as more changes are needed to stop resetting
cross-signing as well.)

res/css/views/settings/_SecureBackupPanel.scss

@@ -34,6 +34,10 @@ limitations under the License.
 
 .mx_SecureBackupPanel_buttonRow {
     margin: 1em 0;
+
+    :nth-child(n + 1) {
+        margin-inline-end: 10px;
+    }
 }
 
 .mx_SecureBackupPanel_statusList {