add GitHub integration functionality to post in a channel when a contributor PR is merged

[josefaidt]

Describe the Feature

Each time a PR is merged in an organization's public repository and the author is not an organization member (i.e. "staff"), post a message in a channel. We will need:

• new table to store PR URL's, time of post, and channel it was posted to
• new field on Guild Configuration table to store channel chosen for these posts
• add/update corresponding GitHub docs (especially if we need additional permissions)
• readonly frontend page (e.g. /admin/github/contributors)
  • this can also be expanded to enable Admins to optionally grant "contributor" label in Discord if we also have their Discord username