Do some basic linting of the PR description

[asmeurer]

In addition to the release notes, we can do some basic linting of the PR description. These wouldn't be things that would fail the build, but would hopefully guide people to writing better descriptions. Some things we can check

• Warn if there is no "fixes #1234" in the description (but say it's OK if there is no corresponding issue)
• Warn if the PR title contains an issue number
• Suggest people to manually post a reference to the PR on the issue
• Make sure there are no commits in the PR that add large files, or add junk files

Any other ideas?