Mark PRs as invalid which have no description

[alexellis]

Expected Behaviour

Mark PRs as invalid which have no description (body) at all.

• Unit tests will be required.
• This needs to be an optional flag as a feature so that people can turn it off / opt-out.

Non-goal: analysing templates to see if they are filled in or not. That is handled via #40

Current Behaviour

Maintainers have to step in to comment and ask for details of the change which is really quite frustrating especially when there is a contribution guide.

[burtonr]

I'll look in to this.

It will be a bit more involved than just if len(pr.Body) < 1 since there can be Pull Request templates that have some amount of text filled in automatically.

Thinking out loud, may need to take the PULL_REQUEST_TEMPLATE.md and compare it to the created PR to find if the user has put any additional text.

Should there be a minimum length of the description that needs to be added? Without some sort of limit, it would be possible to submit a PR with Done in it...

[alexellis]

The scope of this PR is only for blank PRs. There is a separate issue which is blocked due to complexity about template analysis.