Add role attribute to PostStream item for accessibility

[claudiushenrichs]

Changes proposed in this pull request:
The PostStream document structure of a discussion is currently failing the "Elements with an ARIA [role] that require children to contain a specific [role] are missing some or all of those required children." accessibility test. The parent PostStream

container has the "feed" ARIA role which requires children to have the "article ARIA role.
This pull request adds role="article" attribute to the PostStream-item.

Reviewers should focus on:
A few lines further down in PostStream.js (around line 92) there is another reference to a container with classes "className="PostStream-item PostStream-afterFirstPost"" - I haven't figured out under which circumstances this container gets inserted, but it might also require the role=article. If anyone can shed some light on whether this is also a child within the PostStream feed parent container.

Screenshot

Necessity

• Has the problem that is being solved here been clearly explained?
• If applicable, have various options for solving this problem been considered? - article ARIA role is the only one allowed per documentation. Semantically more correct would be something like role="comment", but whom am I to judge.
• For core PRs, does this need to be in core, or could it be in an extension?
• Are we willing to maintain this for years / potentially forever?

Confirmed

• Frontend changes: tested on a local Flarum installation.
• Backend changes: tests are green (run composer test).
• Core developer confirmed locally this works as intended.
• Tests have been added, or are not appropriate here.