Migrate to GitHub actions

This includes some refactors to the build steps. High level items:

* Add variables for DOCKER_IMAGE, S3_RELEASE_BUCKET to allow for forks
  of OPA to re-use the GitHub actions with their own s3 buckets and
  docker orgs/image names.

* Unify the release build steps to use `make release` and the binaries
  being located under `_release/$(VERSION)`. All CI targets now rely
  on binaries being in that `RELEASE_DIR`, including image building
  steps The `make build` target is unaffected.

* Add a wrapper to allow the CI to run the various golang target
  stages separately, but sharing the same docker configuration.

* Conditionally specify `-it` for docker run commands based on whether
  A tty is available.

* Added scripts to automate drafting a release with binary assets vi
  the `hub` CLI.

* The release process triggered on a tag being pushed will now use the
  same binaries from `make release` for the docker images as well as
  the ones attached to the release (which are available under
  https://openpolicyagent.org/downloads/).

The actions themselves are split into 3 workflows:

pull-request.yaml:
  Triggers on pull requests. This will run all the normal tests/checks
  as before on Travis, however they are now split into separate jobs.
  In addition to what was done on Travis we will now have Codecov
  results included.

post-merge.yaml:
  Triggers after a change is pushed to master. This will run tests and
  build+publish the `edge` and `dev` artifacts to dockerhub and s3.

post-tag.yaml:
  Triggers after a tag has been pushed. Similar to post-merge.yaml it
  will run tests and build+publish release artifacts (for the tagged
  version). It will also create a draft release on GitHub with the
  same artifacts and notes from the CHANGELOG.md. If a release already
  exists it will be updated to include the assets, however the release
  notes will _not_ be added.

The RELEASE.md steps have been updated and include notes on the new
steps.

Signed-off-by: Patrick East <east.patrick@gmail.com>