GitHub Actions: add deployment to GH releases and S3

[dyfer]

Purpose and Motivation

Further implementation of #5261

This PR adds deployment steps for our GitHub Actions workflow, making it pretty much complete for Linux and macOS.

Here's an example of a build from this branch: commit and tag.
(it seems that the release artifacts have been since overwritten by Travis... but I can attest they were uploaded properly beforehand)

Here are some implementation notes for a possible discussion before merging:

• both GH release deployment for releases, as well as S3 upload for all commits is implemented in a single job
• the deployment job runs either in the main repo, or for tagged commits (in all repos/forks)
• for GH release uploads I used softprops' action, since the most popular action for that purpose seems unmaintained...
  • this action sets the release author to "github-actions" (previously it was set to @scztt )
  • I've set it to create "draft release", since we need to usually perform some manual tasks before making the release official (namely sign macOS binaries)
  • the action uses emoji in its console output. Probably not a big deal, but I wanted to point this out... 🤷‍♂️
• s3 deployment is triggered for every commit (but not on tags) on the main repo; it checks for existence of the secret key in the repository settings and thus will not be triggered when running in a fork

Types of changes

• New feature

To-do list

• Code is tested
• All tests are passing
• Updated documentation
• This PR is ready for review

[mossheim]

thanks!