Flow123d installation

[jbrezmorf]

• separate creation of the install packages based on the full name of the image passed as a parameter
• automatically create install packages for ci-gnu images, store them on astra
• make github release script for:
  • creating git tag
  • release build and tests
  • image in flow123d/flow123d repository, appropriate installs
  • publication of the release on github
  • (optional) publication on pages
• rebuild install of 3.1.0 branch

[jbrezmorf]

The last step is complicated be a fact, that installation package contains at least reference to the docker image it uses.
For developer releases it depends on ci-gnu images, while release packages must depend on flow123:version images
Therefore we need separate release build on jenkinse but that can be triggered by the gihub release script