Large number of TypeScript errors in strict mode

[jtojnar]

If I enable strict mode and run npm run --prefix client check:js:type, I get 290 errors . Some of these might be just a false positives but the overwhelming number might hide actual bugs.

The easiest will probably be fixing them gradually like in #1544. Help appreciated.

Enabling strict mode

--- a/client/tsconfig.json
+++ b/client/tsconfig.json
@@ -7,8 +7,7 @@
     "resolveJsonModule": true,
     "esModuleInterop": true,
     "forceConsistentCasingInFileNames": true,
-    "noImplicitAny": true,
-    "strict": false,
+    "strict": true,
     "skipLibCheck": true
   }
 }

(strict implies noImplicitAny.)