Refactor the Python library handling process

[vojtechjelinek]

Currently, various Python libraries that need to be installed are specified all over the place. The ones that are required for production are mentioned in requirements.in, the ones that are required for DevOps are mentioned in install_third_party_libs.py (in three places), some other DevOps libraries are also mentioned in manifest.json. Similar issues are with frontend libraries but there we are slowly moving towards using only package.json (some remaining libraries are mentioned in manifest.json, we should be able to get rid of those after the Angular migration).

This issue should be solved by using one common approach for all types of backed (Python) libraries. That doesn't mean that all libraries should be in one exact place (tools and server libs need to be separated), but it means that the process for installing all the libraries should be the same. One option might be to introduce some kind of requirements-tools.in that will list all the reqs for tools.

[U8NWXD]

Done by #15602