Migrate to GitHub Actions

[jGleitz]

The main reason for this Pull Request is that Travis currently takes 30-90 minutes until it starts a commit build. This PR migrates to GitHub actions, which start almost instantly. They are also better integrated with GitHub.

I tested that:

• the CI builds and tests the code.
• the CI uploads the nightly update site if building master.
• the CI uploads the release update site if building a tag starting with releases/.
• when uploading Update Sites, the target folder is cleared, but files outside the target folder are retained.
• the CI takes approximately the same time as on Travis.

For reference, see the Actions in my fork and my test update site.

Differences:

• The configuration mutes some Maven messages that contain no relevant information but clutter the already huge log.
• No email or Slack notifications are configured. GitHub notifies users about failed runs (per default, can be configured in the settings). I think that is enough. Adding Slack notifications would be easy, though.
• The PR also adds the Maven Wrapper to make builds more reproducible.
• The CI is configured to only react to pushes to master, PRs or created tags.

I noticed two bugs:

• GitHub actions messes up Maven’s coloured output, leading to most messages being blue instead of white. I still think that the coloured output is helpful, so I left it as it is.
• GitHub sometimes does not show the full log (or no log) while running. It always shows the full log when the job is completed.

To make the CI work, @HeikoKlare needs to generate an SSH key, add the public key as a Deploy Key with write privileges to the Update Site repository, and add a secret called UPDATE_SITE_DEPLOY_KEY containing the private key to this repository.

[HeikoKlare]

I have added a deploy key and the according secret. They should also work for all other projects of vitruv-tools. As soon as we have merged the other pending PRs, I will also merge this one.

[jGleitz]

@HeikoKlare you will have to disable the required Travis build before merging this PR, and make the GitHub Actions build required after merging this PR.