Start running builds via Actions

[mbohal]

Migrate from Jenkins to GH action:

• run tests for all supported versions of PHP and nightly
• add build shields to README.md
• produce install and update wizzards
• migrate install and update to GH releases section
• update file RELEASING.md

[mbohal]

Apparently Travis i not in the best shape after the takeover in the early 2019. We need to come up with a decision about what service to use

[mbohal]

We will use GH actions

[mbohal]

@DBJRdev All release packages are presently located at http://epartool.download.felis.visionapps.cz/. What should we do about them?

Options:

• take them offline and make new versions available in GitHub releases section
• move them over to GitHub releases section and put some redirect to http://epartool.download.felis.visionapps.cz/

[DBJRdev]

From our side you don't have to provide them anymore at any separate spot than Github. For our users, we upload the installation zips anyway to our tooldoku.dbjr.de server. Besides, the ePartool web downloader script looks for https://tooldoku.dbjr.de/external/epartool-latest.zip, which we update manually with the most recent version so far.