Upgrade roc-package-web-app to use Koa@2

[pmrukot]

Summary

• Updated repository to use newest roc-repo version and jest, roc.config.js was adjusted
• Koa@1 -> Koa@2
• Updated all packages of roc-package-web-app to be Koa@2 compatible
• Middlewares were rewritten to be async functions
• koa-accesslog has been deleted due to incompatibility, now the package uses local implementation (ref src/app/middlewares/accesslog.js)
• Moved middlewares to src/app/middlewares
• koa-errors -> koa-error
• Added some middleware tests

This is a breaking change, middlewares need to be now defined as async functions instead of generators. An API did not change so Im not sure if there are any changes required in packages like: roc-package-web-app-dev, roc-package-web-app-react(-dev). Upgrading those packages to use this one would be also a breaking change.

However, the package should not break the current apps - just throw a warning that generators will be deprecated in Koa v3.

Only tested it with examples/web-app/client-server, however more advanced test cases might be needed.

Had problems with testing createServer since it was breaking Jest and stoped executing test cases.

Need some help with actions required (if there are any regarding other packages) and prerelease process/plan. And review if the changes are sane of course 🙃

[lizell]

Code looks good. It is of course implied, but maybe add a note that we now require Node 8.

I would also love to have the accesslog middleware behind a config toggle.