Fix: [AzurePipelines] switch the CI / CD to CMake too

This also means dropping Debian/jessie, as it has a CMake that is
too old (3.0), with no real path to upgrade.

Fix: [AzurePipelines] in case of a Pull Request, use that number (prefix with 'pr') as branchname

Otherwise the branch name is always 'merge', which is not really
useful or verbose.

Add: [AzurePipeline] introducing a release pipeline

This release pipeline creates all the official release binaries,
and publishes them as artifacts. Currently it can only produce
nightlies and custom builds; stable/testing release binaries are
untested.

This commit also splits up the pipeline in small bits, to both
improve readability, and to share code with the CI pipeline where
possible.