Move build from TeamCity to GitHub Actions (HAST-290)

[Piedone]

Re-implement our current TeamCity build with a GitHub Actions workflows.

• Extend build-dotnet with testing, to have a new, build-and-test-dotnet workflow. This should just not fail if there are no tests, like build-and-test-orchard-core already does (i.e. as test-dotnet does) once Lombiq/GitHub-Actions#118 is merged. Then, we can deprecate build-dotnet.
• Use larger runners for PR builds, but the free standard runners for dev and client builds.
• Also change the build badge in the root Readme.

Jira issue