Downgrade CKEditor to 19 so we can build on node 10

frontend/package.json

@@ -31,7 +31,7 @@
     "typescript": "^3.9.3"
   },
   "dependencies": {
-    "@ckeditor/ckeditor5-build-classic": "^23.0.0",
+    "@ckeditor/ckeditor5-build-classic": "^19.0.0",
     "@types/d3": "^5.7.2",
     "d3": "^6.2.0",
     "sirv-cli": "^1.0.0",

frontend/yarn.lock

@@ -278,10 +278,10 @@
   resolved "https://registry.yarnpkg.com/@bcoe/v8-coverage/-/v8-coverage-0.2.3.tgz#75a2e8b51cb758a7553d6804a5932d7aace75c39"
   integrity sha512-0hYQ8SB4Db5zvZB4axdMHGwEaQjkZzFjQiN9LVYvIFB2nSUHW9tYpxWriPrWDASIxiaXax83REcLxuSdnGPZtw==
 
-"@ckeditor/ckeditor5-build-classic@^23.0.0":
-  version "23.0.0"
-  resolved "https://registry.yarnpkg.com/@ckeditor/ckeditor5-build-classic/-/ckeditor5-build-classic-23.0.0.tgz#9b4ae5dc470e9ae4be14672b49d05d21887e2d81"
-  integrity sha512-9Pgb9OAPnx1577WDYizEuOFHmLyXkwU8UxneyP4Zkr9mrJUEL0w0uVgQhiB8DYlAAArHXeFasSogwFvLX6xBtw==
+"@ckeditor/ckeditor5-build-classic@^19.0.0":
+  version "19.0.2"
+  resolved "https://registry.yarnpkg.com/@ckeditor/ckeditor5-build-classic/-/ckeditor5-build-classic-19.0.2.tgz#ecab0e38d3f6fa7560f6937d18f711243b20b978"
+  integrity sha512-yz2M9Q6jMy9zF+JiQiSoaG/m7Eu48H+Yi3FgtVGDFpLDqGUJntNsW26g3qZld5Pa4pgVPRmDcfKHzLTWAo3iRg==
 
 "@cnakazawa/watch@^1.0.3":
   version "1.0.4"