Fix plurals in translations

Weblate doesn't understand counterpart's multi-level format for
plurals and helpfully mangles them to be dot-separated keys when it
saves the translation file. Store the source files with pipe
separators and convert to counterpart format at build time.

src/i18n/strings/ja.json

@@ -44,8 +44,8 @@
     "Only people who have been invited": "この部屋に招待された人のみ参加可能",
     "Hide removed messages": "削除された発言の印を表示しない",
     "Always show message timestamps": "発言時刻を常に表示",
-    "%(count)s new messages.one": "新しい発言 %(count)s",
-    "%(count)s new messages.other": "新しい発言 %(count)s",
+    "%(count)s new messages|one": "新しい発言 %(count)s",
+    "%(count)s new messages|other": "新しい発言 %(count)s",
     "Don't send typing notifications": "文字入力中であることを公表しない",
     "Filter room members": "参加者検索",
     "Send a message (unencrypted)": "ここに送信文を入力 (暗号化なし)",