Merge public ihasmail: folder reordering, full-screen composer, Dutch update

- Reorder folders by dragging, with special folders first (#402, #405).
- Open the composer full screen, as a setting (#401, #404).
- Dutch translation update (#403).

FEATURES.md stays deleted here, as in bb25355.

web/src/locales/zh-Hans.ts

@@ -747,6 +747,7 @@ export const catalog: Catalog = {
     "Attachment reminder": "附件提醒",
     "Warn when the message mentions an attachment but none is attached.": "邮件提到附件但未添加时提醒。",
     "Spell check while typing": "输入时检查拼写",
+    "Open the composer full screen": "全屏写邮件",
     "Confirm before deleting": "删除前确认",
     "Show message snippets": "显示邮件摘要",
     "Preview the first line of each message in the list.": "在列表中显示每封邮件的首行。",