FR: Separating the packages to submodules

[akauppi]

Instead of packages/{app|backend} those will be linked from separate GitHub repos, thus detaching the version control of them from the building/deployment framework, completely.

Definition of Done:

• At this point, some framework specificity may still remain on the side of the submodules, but not in the framework.
• Two demo projects can be swapped, by just redirecting the packages/ links.

Later:

• Also the remaining framework aspects within the packages are somehow moved to framework side