Add linting for unexpected spacing in output code blocks.

[hexylena]

here is the offending code block, we need to detect this in output HTML, and complain their markdown is improperly indented. xref https://github.com/galaxyproject/training-material/issues/4853

<code> https://zenodo.org/record/4274812/files/bulk_bottom.fastq.gz
 https://zenodo.org/record/4274812/files/bulk_top.fastq.gz
 https://zenodo.org/record/4274812/files/rhizosphere_bottom.fastq.gz
 https://zenodo.org/record/4274812/files/rhizosphere_top.fastq.gz
</code>

it's not possible to tell this from the markdown side, whether or not this is what the user meant.

If we can disable support for space indented code blocks that would honestly be a win, and ensure everyone only uses fenced code blocks.

[hexylena]

@AnnaSyme found more.

Hi all, in the VGP assembly tutorial, there seems to be spaces before some (all?) of the zenodo links to files. Thus when people paste these in to Galaxy to upload, it doesn't recognize them as URLS, just as a text string in a pasted entry. I can edit the tutorial in github to remove these spaces unless anyone thinks this is a bad idea? Tutorial: https://training.galaxyproject.org/training-material/topics/assembly/tutorials/vgp_genome_assembly/tutorial.html#hybrid-scaffolding-with-bionano-optical-maps