Support special Markdown attribute syntax

[NickSto]

The .eu hub uses a special remark-attr-like syntax on some of its pages. We don't currently support remark-attr or anything like it.

Note: it's Kramdown's Inline Attribute Lists

Either we need to support it (ideally) or we'd have to manually convert those pages.

[NickSto]

Manual conversion might be a bit of a lift.

A quick grep shows at least 890 instances of it across 91 files:

[Tue Jan 18] me@aziz: ~/galaxy/code/eu-hub
$ git grep -FIl '{:' '*.md' | wc -l
91
[Tue Jan 18] me@aziz: ~/galaxy/code/eu-hub
$ git grep -FI '{:' '*.md' | wc -l
890

[NickSto]

Update: So this syntax actually does seem to work sometimes? I didn't think remark-attr was anything more than barely functional here but I guess it's (conditionally) doing its job?

When does it work?

It seems to only work on some pages, which is probably how I got the impression it doesn't work.

Based on the following tests, it seems it's vue-remark which can't handle it:

Works?	Collection	parser	File
works	Insert	source-filesystem	content/bare/usegalaxy-eu/notices.md
works	Insert	source-filesystem	content/footers/eu.md
works	Article	source-filesystem	content/get-started/index.md
works	Article	source-filesystem	content/events/2021-10-sars-cov-2-training/index.md
broken	BareArticle	vue-remark	content/bare/usegalaxy-eu/metabolomics/index.md
broken	VueArticle	vue-remark	content/events/gcc2022/index.md (manual test)

Whitespace issue

The only hitch is that it seems you need a between the {: and certain special characters like .. Otherwise it won't parse anything until there's a space. That is, in {:.fa .fa-list-ul}, it will ignore the .fa but add the .fa-list-ul. But regular letters seem fine: {:target="_blank"} works. This means a lot of the current .eu content will have to be modified, but minimally