Use flex to center #message-box (#1271)

* Use flex to center #message-box

- Resolves #1270
- See details on the issue mentioned.

* Use existing .center

* Remove center rule at non-center selector

manage/import-export.js

@@ -209,6 +209,7 @@ async function importFromString(jsonString) {
       contents: $create('#import', report.length ? report : t('importReportUnchanged')),
       buttons: [t('confirmClose'), numChanged && t('undo')],
       onshow: bindClick,
+      className: 'center',
     })
       .then(({button}) => {
         if (button === 1) {

manage/updater-ui.js

@@ -224,6 +224,7 @@ async function showUpdateHistory(event) {
   messageBoxProxy.show({
     title: t('updateCheckHistory'),
     contents: log,
+    className: 'center',
     blockScroll: true,
     buttons: [
       t('confirmOK'),