Add accessibility report to the merge request reports tab

What does this MR do and why?

Adds AccessibilityProvider, AccessibilityNavItem, AccessibilityPage, and AccessibilityContent, following the code quality report pattern, plus a View report button on the accessibility widget. The error grouping and summary logic moves into a shared utils.js used by both the widget and the report, which also fixes the widget reporting a load failure while the report is still being parsed.

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