[A11Y] Make checkboxes focusable

[davwheat]

Fixes #2973

Changes proposed in this pull request:

• Make checkbox inputs appear over their display elements so that they are accessible to mobile screen-readers
• Add aria-hidden to the purely visual checkbox elements
• Restore browser focus rings to the checkbox display elements

Reviewers should focus on:

Nothing, really.

Screenshot

Ef2UclMY.mp4

Confirmed

• Frontend changes: tested on a local Flarum installation.

[davwheat]

Confirmed that Talkback on Android works correctly with the checkboxes.

Interestingly, aria-hidden=true needs to be set as opposed to just aria-hidden for talkback to pick up on it. I'm going to need to fix some other bits of core to reflect that.