update changelog and website checks

[alvin-huang]

This PR splits out the website/ check to only run on PR opened while the changelog check runs on opened, synchronize (new commits) and labeled and also changes it to a status check that fails when there is no pr/no-changelog label applied nor is there a .changelog/$pr_number.txt file.

I also changed the on trigger to be pull_request for the changelog check because it no longer needs a token secret to post to the PR so it doesn't need to only run on pull_request_target

[hashicorp-ci]

🤔 Double check that this PR does not require a changelog entry in the .changelog directory. Reference

[alvin-huang]

🤔 Double check that this PR does not require a changelog entry in the .changelog directory. Reference

This will go away after this gets merged, it's running the old check in the master branch.

[dnephin]

LGTM!

One question, but not blocking, probably just a minor optimization.

[dnephin]

Can we omit this now that it uses on: pull_request instead of target?

[alvin-huang]

I can test but I don't think so, we still need to checkout all branches to compare the HEAD to the base ref (which wouldn't be checked out if we just checkout HEAD).

[hashicorp-ci]

🍒 If backport labels were added before merging, cherry-picking will start automatically.

To retroactively trigger a backport after merging, add backport labels and re-run https://circleci.com/gh/hashicorp/consul/333004.