Check for pull requests made from non-forks

[chrisburr]

It would be nice to detect and then comment on PRs with an explanation of why PRs need to be made from forks.

[beckermr]

That's a great idea. We should do this in the webservices bot though.

[beckermr]

This bot already handles PR events for linting and commands. I think it should be an easy addition to check what repo the head is in.

[beckermr]

I think if you put the check here: https://github.com/conda-forge/conda-forge-webservices/blob/master/conda_forge_webservices/commands.py#L73 it will catch things easily.

[jakirkham]

Just another thought, if we make this a separate status and make it required, this would ensure that people can't push non-checked commits to branches (thus protecting against a lot of the bad outcomes here)

[beckermr]

cc @conda-forge/core