Re-enable fetchstyle plugin and request it from s3 repository

[oguzkocer]

Since the source code for fetchstyle plugin is heavily outdated, I couldn't fully update it in the time I allotted for it. I actually successfully published the plugin to S3, but there was something wrong with the groovy dependency, so it didn't work when I tried to consume it. So, for now, I've manually ported the fetchstyle plugin from jcenter to our s3 repository.

Edit: I realized there was one last thing I forgot to revert from https://github.com/Automattic/stories-android/pull/674 and that's to re-enable the checkstyle task in CI. I've done this in 359ba35f1973912d414996a2dd99c0a60304243c. This task should have never been commented out because it had nothing to do with the fetchstyle plugin, but I didn't recognize this during that day's craziness.

To test:

• Run ./gradlew tasks --all | grep downloadConfigs to verify the fetchstyle plugin tasks are available. Or you can run the downloadConfigs with ./gradlew downloadConfigs.
• Run ./gradlew --scan and open the scan. In the Build Dependencies section, use the search functionality to find the fetchstyle dependency and verify it's repository is our s3 repo and not jcenter

[peril-automattic[bot]]

You can test the changes on this Pull Request by downloading the APK here.