Refactor CI workflows

[mcserep]

This PR separates the CI into multiple workflows, closing #587. Tasks:

• [x] Eliminate redundancy in Ubuntu APT package installation between the build and parse jobs.
• [x] Eliminate redundancy of PostgreSQL / SQLite3 based CMake configuration in the build job.
• [x] Eliminate redundancy of PostgreSQL / SQLite3 based project parsing and testing in the parse job.
• [x] Decompose docker job into separate workflow.
• [x] Decompose tarball job into separate workflow.

The PR also removes the Ubuntu 18.04 jobs, as it has been deprecated in GitHub Actions, and the image is not even available anymore. Build instructions should be updated by #580 for this matter.

[mcserep]

@intjftw I completed the PR, please review it when you have time.