Add frontend install and build to bootstrap script

Now you don’t need to remember what the two commands are and when you need to
run them, you can just run the bootstrap script instead.

Makes sense to have them here since the `pip install` is also in here.

scripts/bootstrap.sh

@@ -27,3 +27,5 @@ fi
 
 # Install Python development dependencies
 pip3 install -r requirements_for_test.txt
+
+npm install && npm run build