i18n: localize the editor toolbar in all 23 locales

The rich-text editor was the last hardcoded-English surface in the
composer: 21 tooltip titles, the eight table-menu entries, the "Remove
color" entry and the table size picker's "Pick size" label were plain
strings while every other menu in the app is localized.

All of them now come from a new email_composer.toolbar namespace,
translated into all 23 locales using each platform's established
editor terminology (Word/Docs conventions - de "Formatierung löschen",
ar "مسح التنسيق", ja "書式をクリア", ...). The link prompt stays "URL",
which is the same term in every language.

Tooltips and self-sizing dropdowns have no width constraints, so longer
translations are safe everywhere.

locales/nl/common.json

@@ -686,6 +686,38 @@
     "recipient_name_placeholder": "Weergavenaam",
     "autocomplete_search_server": "Op de server zoeken",
     "autocomplete_searching": "Bezig met zoeken...",
+    "toolbar": {
+      "bold": "Vet",
+      "italic": "Cursief",
+      "underline": "Onderstrepen",
+      "strikethrough": "Doorhalen",
+      "text_color": "Tekstkleur",
+      "remove_color": "Kleur verwijderen",
+      "heading_1": "Kop 1",
+      "heading_2": "Kop 2",
+      "bullet_list": "Opsommingslijst",
+      "ordered_list": "Genummerde lijst",
+      "quote": "Citaat",
+      "code_block": "Codeblok",
+      "align_left": "Links uitlijnen",
+      "align_center": "Centreren",
+      "align_right": "Rechts uitlijnen",
+      "text_direction": "Tekstrichting (RTL/LTR)",
+      "link": "Link",
+      "table": "Tabel",
+      "clear_formatting": "Opmaak wissen",
+      "undo": "Ongedaan maken",
+      "redo": "Opnieuw",
+      "add_row_above": "Rij erboven toevoegen",
+      "add_row_below": "Rij eronder toevoegen",
+      "add_column_before": "Kolom ervoor toevoegen",
+      "add_column_after": "Kolom erna toevoegen",
+      "delete_row": "Rij verwijderen",
+      "delete_column": "Kolom verwijderen",
+      "toggle_header_row": "Koprij aan/uit",
+      "delete_table": "Tabel verwijderen",
+      "pick_size": "Grootte kiezen"
+    },
     "send_filing_warning": "Verzonden - maar het opruimen daarna is mislukt, mogelijk blijft een oud concept staan."
   },
   "confirm_dialog": {