Remove and then lint for reference style links

Problem to solve

As our styleguide states, we should not use reference-style links anymore:

OK:

[this is a link](example.com)

Bad:

[this is a link][reference-link]

[reference-link]: example.com

Let's clean all these up, and then add a Vale rule to prevent any new ones from being added.

Currently, there are 993 results, reduced to 921 with the first cleanup MR listed in the related MRs. See the failed job in !28281 (merged) (WIP introducing the Vale rule) for a list of all results.

Edit: Down to 403 already! Thanks everyone!

Edit: 212!

Edit: 84!

Edit: 26! TWENTY! SIX!