feat(broadcasts): add admin broadcast page and user opt-out switch

Admin page /admin/broadcasts to write, test and send broadcasts in both
languages; confirms the recipient count before send. History shows the
counts. A new switch in settings lets users opt out of platform mails;
account mails always come. Subject and body are plain text only.

public/i18n/en.json

@@ -130,7 +130,9 @@
     "PROFILE_TILES_HINT": "Choose what your profile shows, and where: open your profile and click \"Customize\".",
     "FAVORITE_PLATFORM": "Favorite platform",
     "FAVORITE_PLATFORM_HINT": "Selected first when you add a game on more than one platform.",
-    "FAVORITE_PLATFORM_NONE": "None"
+    "FAVORITE_PLATFORM_NONE": "None",
+    "PLATFORM_MAILS": "Mails from the platform",
+    "PLATFORM_MAILS_HINT": "News and announcements by mail. Mails about your account always come."
   },
   "VALIDATION": {
     "EMAIL_REQUIRED": "Email is required.",
@@ -304,6 +306,7 @@
     "ADMIN_HINTS": "Hints",
     "WHATS_NEW": "What's new",
     "ADMIN_NEWS": "News",
+    "ADMIN_BROADCASTS": "Mail",
     "PENDING_REQUESTS_one": "{{count}} registration waits",
     "PENDING_REQUESTS_other": "{{count}} registrations wait"
   },
@@ -691,6 +694,35 @@
     "LOAD_ERROR": "The entries could not be loaded.",
     "NO_ACCESS": "Only admins can open this page."
   },
+  "ADMIN_BROADCAST": {
+    "NEW": "New mail",
+    "SUBJECT_EN": "Subject (English)",
+    "SUBJECT_DE": "Subject (German)",
+    "BODY_EN": "Text (English)",
+    "BODY_DE": "Text (German)",
+    "TEXT_HELP": "Plain text, both languages. Subject max 150, text max 5000 characters. A footer with the opt-out link is added.",
+    "RECIPIENTS": "Recipients",
+    "ALL": "All active users",
+    "PICKED": "Picked users",
+    "SEARCH": "Search email or name",
+    "PICKED_COUNT": "{{count}} picked",
+    "REMOVE": "Remove",
+    "TEST": "Send test to me",
+    "TEST_SENT": "Test mail queued for you.",
+    "SEND": "Send...",
+    "CONFIRM_TITLE": "Send this mail?",
+    "CONFIRM_TEXT": "Send to {{count}} users? This cannot be undone.",
+    "CONFIRM": "Send",
+    "CANCEL": "Cancel",
+    "NO_RECIPIENTS": "No recipient: nobody active with mails on.",
+    "SENT": "Mail queued for {{count}} users.",
+    "ERROR": "The action failed.",
+    "LIST": "Sent mails",
+    "COUNTS": "{{recipients}} recipients, {{sent}} sent, {{failed}} failed",
+    "EMPTY": "No mails sent yet.",
+    "LOAD_ERROR": "The mails could not be loaded.",
+    "NO_ACCESS": "Only admins can open this page."
+  },
   "TILES": {
     "CUSTOMIZE": "Customize",
     "EDIT_HINT": "Move, resize or hide the tiles. Nothing is saved before \"Done\".",