fix(i18n): add missing dialogs.share_folder translation key

The share dialog for folders was failing because dialogs.share_folder
key was missing from all locale files. The contextMenus.js code
references this key when opening the share dialog for folders:

  window.i18n.t('dialogs.share_folder')

Added the missing key to all 14 locale files with appropriate
translations for each language.

Fixes #189

static/locales/ja.json

@@ -229,6 +229,7 @@
     "confirm_delete_share": "共有リンクを削除",
     "confirm_delete_share_msg": "この共有リンクを削除しますか？",
     "share_file": "ファイルを共有",
+    "share_folder": "フォルダを共有",
     "existing_shares": "既存の共有",
     "share_options": "共有オプション",
     "password": "パスワード",