Signed-off-by: Vitalii Koshura <lestat.de.lionkur@gmail.com>

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>

Signed-off-by: Vitalii Koshura <lestat.de.lionkur@gmail.com>

Signed-off-by: Vitalii Koshura <lestat.de.lionkur@gmail.com>

Signed-off-by: Vitalii Koshura <lestat.de.lionkur@gmail.com>

Update README.md and CONTRIBUTING.md with updated links and information about how to connect via slack or email

Signed-off-by: Vitalii Koshura <lestat.de.lionkur@gmail.com>

Made link to gnu.org https

- An email is send only to those adresses given in .travis.yml
- Only the master branch is build
- The compiler gcc and clang are used
- The components libraries, server and client/manager are build separately
- The README.md just displays the current build status on the github mirror