Major refactoring

* Sequencing mode-related changes:
  * allowed sequencing modes in Snakemake input table changed from `paired_end` and `single_end` to `pe` and `se`, respectively
  * remove sequencing mode from output paths for each rule
  * corresponding wild cards removed entirely from all rules that do not depend on sequencing mode (currently all rules that are defined in the main `Snakefile` in the project root directory)
  * where absolutely necessary, sequencing mode is added as part of output file or directory instead
  * remove dependency of sequencing mode for rule for `FastQC`; now runs separately for each strand
* Changes related to MultiQC and output file/directory structure
  * moving and renaming outputs for MultiQC is no longer required
  * code to create MultiQC custom config externalized into script `scripts/rhea_multiqc_config.py`
  * add MultiQC output files with deterministic output to md5 sum checks performed during execution of `tests/test_integration_workflow/test.{local,slurm}.sh`
  * output filenames for each rule now follow this general structure: `samples/{sample_name}/{rule}/{output_file}`
  * change log directory structure matches results directory structure
* Miscellaneous changes
  * consistent, PEP8-compliant formatting in most parts, including Snakemake files, where allowed
  * remove rule `extract_decoys_salmon`; equivalent file `chrName.txt` produced by `star_index` is used instead
  * add rule `start` which copies sample data to the results directory and enforces uniform naming
  * refactoring of ALFA rules and modification of the CI/CD test to ensure compatibility