[CI] replace Travis and AppVeyor with Github Actions

This consolidates the different CI builds on one platform that provides support for the major three operating systems we build on (Linux, Windows, MacOS). The nomenclature and syntax is a bit different but it has the same functionality.
This commit introduces the Linux and Windows based workflows. All the build artifacts can now be downloaded from [bintray](https://bintray.com/beta/#/boinc/boinc-ci?tab=files

). The CI workflows run for every pull request, every merge into the master branch and every Sunday around 12:00 (GMT).
Every first Sunday in a month a cleanup Script is run to delete old artifacts from Bintray. Artifacts from closed/merged pull requests get deleted then. Artifacts from weekly builds and merges into the master branch older than 6 months get deleted too.

Co-authored-by: Vitalii Koshura <lestat.de.lionkur@gmail.com>