Add missing a11y attributes to "Mark as read" button on DiscussionListItem

[davwheat]

Changes proposed in this pull request:
This PR adds missing accessibility attributes to the "Mark as read" button on DiscussionListItem.

<span>s are inherently inaccessible to keyboard users and unfocusable. Using an onClick on them without any other attributes indicating that this performs an action is bad practice as it means keyboard navigators and screen-readers cannot access the elements, so never know that the buttons exist.

Assigning unfocusable elements with a tabindex="0" allow them to be focuses, and role="button" lets screen-readers know that it is a button-like element.

Reviewers should focus on:

Other locations where issues like this may be present: they may be able to be bundled into one whole PR.

Screenshot

Confirmed

• Frontend changes: tested on a local Flarum installation.
• Backend changes: tests are green (run composer test). (N/A)