feat: support importing emails from .zip archives

locales/en/common.json

@@ -270,7 +270,7 @@
     "print": "Print",
     "view_source": "View source",
     "export_email": "Export as .eml",
-    "import_email": "Import .eml",
+    "import_email": "Import .eml or .zip",
     "keyboard_shortcuts": "Keyboard shortcuts (?)",
     "email_source": "Email Source",
     "draft_banner": "This message is a draft",
@@ -1794,7 +1794,7 @@
     "new_subfolder": "New subfolder...",
     "new_folder": "New folder...",
     "rename": "Rename...",
-    "import_email": "Import .eml...",
+    "import_email": "Import .eml or .zip...",
     "empty_folder": "Empty folder",
     "empty_folder_generic": "Empty folder",
     "delete_folder": "Delete folder",