[TASK] Add trait providing standard implementation of Commentable

[JakeQZ]

Resolves #813.

[coveralls]

coverage: 52.257% (+0.2%) from 52.048%
when pulling 65bb1e7 on task/commentable-trait
into ece5633 on main.

[oliverklee]

Maybe we can describe the effects instead of focusing on the methods:

Suggested change

	public function getCommentsAfterEmptyArrayOfCommentsAddedReturnsOriginalComments(array $comments): void
	public function setCommentsWithEmptyArrayKeepsOriginalCommentsUnchanged(array $comments): void

[JakeQZ]

Apart from getCommentsInitiallyReturnsEmptyArray(), all the test methods focus on either addComments() (first group) or setComments() (last group). So perhaps they all be renamed like addCommentsDoesXYZ() and setCommentsDoesXYZ - WDYT?

[oliverklee]

Yes, I'd like that!

[JakeQZ]

Done.

[oliverklee]

There's a leftover use (which PHP-CS-Fixer is complaining about), and also one open comment.

[JakeQZ]

There's a leftover use (which PHP-CS-Fixer is complaining about), and also one open comment.

I've replied to the open comment with a question. I've fixed the leftover use - I'd forgotton to do git add before amending the commit.