use result file for determining status of a job

- includes numerous related changes
- uses job_metadata.get_section_from_file to read job metadata and return PR
  section
- renames determine_eessi_tarballs to determine_artefacts
  - uses _bot_jobSLURM_JOBID.result file to obtain artefacts
  - it relies on what the script bot/check-build.sh in the target GitHub
    repository determines as artefacts, thus it does need to know anything about
    the target GitHub repository
- renames check_build_status to check_job_status
  - completely overhauled logic which essentially relies on the value of the
    status attribute in the _bot_jobSLURM_JOBID.result file
  - it relies on what the script bot/check-build.sh in the target GitHub
    repository determines as status, thus it does need to know anything about
    the target GitHub repository
- renames upload_tarball to upload_artefact
  - no changes of logic
  - mostly renamed 'tarball' to 'artefact' (except for tarball_prefix* related
    variables; these might be changed in a later PR; a change would also require
    changes to the configurations, app.cfg.example and README.md)
  - also changed 'build_target' to 'payload' and made clear that
    'eessi-VERSION-COMPONENT-OS-ARCH' is just format used within EESSI to
    describe the contents of a payload (or build_target)
- updated uploaded_before
  - no changes of logic
  - uses 'payload' instead of 'build_target'
- updated determine_successful_jobs
  - removes unnecessary function calls: determine_slurm_out
  - removes unnecessary key/value pair in job dictionary: 'slurm_out'
  - switches to using 'artefact' instead of 'eessi_tarball' for variables and
    functions (determine_artefacts)
  - uses renamed and overhauled check_job_status
  - minor polishing of log messages: 'build' -> 'job'
- renames determine_tarballs_to_deploy to determine_artefacts_to_deploy
  - removes mentioning of 'slurm_out' data in job dictionary (docstring)
  - rephrases 'build*target' to 'payload' (docstring)
  - rephrases '(built/eessi) tarballs' to 'artefacts' (docstring, variables, log
    messages)
  - renames 'tb0' to 'artefact'
- updated deploy_built_artefacts
  - uses determine_artefacts_to_deploy instead of determine_tarballs_to_deploy
  - rephrases 'target' to 'payload'
  - uses upload_artefact instead of upload_tarball