Enable TypeScript strict mode

This enables strict mode, but sets strictPropertyInitialization to false,
as "the class-validator DTO pattern described in the documentation is
not compatible with strict property initialization" according to
https://github.com/nestjs/typescript-starter/pull/192

Signed-off-by: David Mehren <git@herrmehren.de>

tsconfig.json

@@ -14,6 +14,7 @@
       "./types",
       "./node_modules/@types"
     ],
-    "strict": false
+    "strict": true,
+    "strictPropertyInitialization": false
   }
 }