Mergeback

A simple github action to merge back master to a development branch.  This action uses an optional environment variable:

• BASE_BRANCH The base branch where master changes should be merged to. Defaults to develop.
• DESC_PATH The path in your repository for the PR body template. Defaults to .github/merge-instructions.md. If this file exists, this will be used for the description of the PR (only if it's not mergable). Environment variables can be used in this file, they will be resolved.
• REVIEWERS An array with the list of reviewers for the PR, if there are merge conflicts
• TEAM_REVIEWERS An array of team slugs for reviewing the PR, if there are merge conflicts
• HEAD_USER (Optional) Github API now has a validation of the head with the format username:ref-name. If HEAD_USER specified, HEAD_ORG is ignored
• HEAD_ORG (Optional) Github API now has a valitation of the head with the format organisation:ref-name.

The action uses GitHub's REST API to create a PR and merge it remotely on GitHub's servers.