ESLint: enforce Angular HTML no-negated-async

https://github.com/angular-eslint/angular-eslint/blob/main/packages/eslint-plugin-template/docs/rules/no-negated-async.md

Prevents us from writing conditions that may pass/fail before an observable has emitted (this is likely to cause unexpected behaviour)

.eslintrc.json

@@ -235,7 +235,6 @@
       ],
       "rules": {
         // todo: re-enable & fix errors
-        "@angular-eslint/template/no-negated-async": "off",
         "@angular-eslint/template/eqeqeq": "off"
       }
     },