Add rule to enforce spacing in curly brackets in JSX children

This required an updated in version of eslint-plugin-react to 7.4.0.

jenkins.sh

@@ -21,9 +21,7 @@ npm run test -- --no-colors
 npm run lintall -- -f checkstyle -o eslint.xml || true
 
 # re-run the linter, excluding any files known to have errors or warnings.
-./node_modules/.bin/eslint --max-warnings 0 \
-    --ignore-path .eslintignore.errorfiles \
-    src test
+npm run lintwithexclusions
 
 # delete the old tarball, if it exists
 rm -f matrix-react-sdk-*.tgz