Add a computed field for the course action element

Let's see

It was a decision I've made to make the computed field returning markup. Initially, I was thinking that it could be LinkItem type. Then I saw sometimes it returns simple markup, not link. Ended by adding a new template. Still deserves some tests

OK, it's ready for review.

Few notes:

• I've made a decision to make the computed field returning markup. Initially, I was thinking that it could be LinkItem type. Then I saw that sometimes it returns simple markup, not link. Ended by adding a new template. Cacheability is still honoured, thanks to https://www.drupal.org/node/3423720 →
• This link wording is ambiguous, sometimes is "start link" on other places is "join link" (see CSS classes). I think we need to stick to one. Which?
• There's variable $htmlClasses (inherited from $html_classes) but not used before this change. I just kept it but I don't think it makes any sense. Maybe I'm missing something...

Nice one, thanks, code changes look good. Let me test this.

Automatically closed - issue fixed for 2 weeks with no activity.