feat: support importing emails from .zip archives

locales/nl/common.json

@@ -270,7 +270,7 @@
     "print": "Afdrukken",
     "view_source": "Bron bekijken",
     "export_email": "Exporteren als .eml",
-    "import_email": ".eml importeren",
+    "import_email": ".eml of .zip importeren",
     "keyboard_shortcuts": "Sneltoetsen (?)",
     "email_source": "E-mailbron",
     "draft_banner": "Dit bericht is een concept",
@@ -1794,7 +1794,7 @@
     "new_subfolder": "Nieuwe submap...",
     "new_folder": "Nieuwe map...",
     "rename": "Hernoemen...",
-    "import_email": ".eml importeren...",
+    "import_email": ".eml of .zip importeren...",
     "empty_folder": "Map leegmaken",
     "empty_folder_generic": "Map leegmaken",
     "delete_folder": "Map verwijderen",