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.

src/utils/limits.js

@@ -33,3 +33,6 @@ export const MAX_HINT_LENGTH = 500;
 // Auth NewsRequest @Size(max = 100) and @Size(max = 2000)
 export const MAX_NEWS_TITLE_LENGTH = 100;
 export const MAX_NEWS_TEXT_LENGTH = 2000;
+// Auth BroadcastRequest @Size(max = 150) and @Size(max = 5000)
+export const MAX_BROADCAST_SUBJECT_LENGTH = 150;
+export const MAX_BROADCAST_BODY_LENGTH = 5000;