Add yarn install to CI

[ananyo2012]

I see @dependabot making some PRs for yarn packages. There is a yarn.lock present at https://github.com/pythonindia/junction/tree/master/junction/static . In case that is being used we need to add that to CI to keep track of dependency updates.

[ananyo2012]

We are using yarn packages for css assets only. For JS we are still using vanilla jquery library. Unless there is linting or tests setup we don't need to run yarn in CI.