Add accessibility attributes to loading spinner

[davwheat]

Changes proposed in this pull request:
After doing some research, it looks like we need a few a11y attributes on the loading spinner.

role="status" seems a little weird at first (at least it did to me), but it's important. W3 describe a status message as:

[a] change in content that is not a change of context, and that provides information to the user on the success or results of an action, on the waiting state of an application, on the progress of a process, or on the existence of errors

This role also sets an implicit aria-live="polite", meaning screenreaders will know that the contents of this element may change in the future, and that they should notify the user of this element being added/removed/changed in real-time.

We also add an accessible label to ensure that screen readers understand what the element is.

Screenshot

Confirmed

• Frontend changes: tested on a local Flarum installation.