Add native support for GitHub & GitLab secret tokens

[JacobDB]

The logic would be something like:

1. Check for $_SERVER["HTTP_X_HUB_SIGNATURE"] (GitHub token)
2. If no GitHub token exists, check for $_SERVER["HTTP_X_GITLAB_TOKEN"] (GitLab token)
3. If no GitLab token exists, check for $_GET["token"]
4. If any of the three tokens exist, compare the first one against the secret specified in config.php
5. If no token exists, proceed with script

I'll submit a pull request for this some time this week.

Documentation:

• https://developer.github.com/webhooks/#payloads
• https://gitlab.com/help/user/project/integrations/webhooks#secret-token

Example:

• https://gist.github.com/milo/daed6e958ea534e4eba3

[JacobDB]

Need to update the screenshots to match the new settings.