[ENH] Use Github actions as a new CI system.

[JakaKokosar]

Issue
Experimentally use Github actions as a new CI system.

Description of changes
Run new CI alongside Travis and Appveyor to test how it behaves.

Known downsides:

Update: Previous running jobs are cancelled if a new commit is pushed to the same PR 👍

Github action is not as feature reach as other systems because it was released not long ago. So I noticed that currently there is no officially supported way to cancel "stale" commits in a certain branch. Potentially this could be a problem. I hope there will soon be support for this. For now, we could use some third-party solutions that use Github public APIs (did not test those yet).
https://github.community/t5/GitHub-Actions/Github-actions-Cancel-redundant-builds/td-p/29549

To see running job go here: https://github.com/biolab/orange3/actions

Linux job (ubuntu 18.04):

• runs coverage for python 3.6 and 3.7.
• spawns two contianers (Postgres and mssql)
• Postgres container comes with needed extensions. Dockerfile is defined in this PR: Add postgres Dockerfile orange-tools#15 and repository with docker image is here: https://hub.docker.com/repository/docker/orangedm/postgres
• Token for codecov upload is here: https://github.com/biolab/orange3/settings/secrets

To-do:

• Complete tests with codecoverage (linux)
• Run tests on Windows and MacOS (tests that need database support will be skipped.)
• Pylint
• Documentation build check

[codecov]

Codecov Report

Merging #4482 into master will increase coverage by 0.37%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #4482      +/-   ##
==========================================
+ Coverage   83.26%   83.64%   +0.37%     
==========================================
  Files         274      279       +5     
  Lines       54851    59180    +4329     
==========================================
+ Hits        45671    49500    +3829     
- Misses       9180     9680     +500

[markotoplak]

This PR already proved its usefulness: OSX tests are failing since merging #4443. They also fail locally for me and @JakaKokosar.