Compatibility message and error handling in content editor

What does this MR do?

This MR fixes issues: #327249 (closed) and #328862 (closed)

Screenshots (strongly suggested)

Screenshot	Comments
	State A: Markdown editor is loaded by default Clicking on Use new editor will take you to State B (in case of success) or State D (in case of error) Note: Error state can be simulated by blocking all requests going to [project_path]/-/wikis/[wiki_page_path]/preview_markdown
	State B: Content editor loads successfully Clicking on Switch to old editor will show a modal confirming the change as seen in State C.
	State C: Modal confirmation to switch back to markdown editor Keep editing closes the modal and Switch to old editor will switch back to State A.
	State D: Content editor fails to load An error alert is shown on top with a Reload CTA. The editor is shown in a perpetual loading state. Save changes button at the bottom is disabled. Clicking Switch to old editor here will switch back to State A without showing a modal.

Does this MR meet the acceptance criteria?

Conformity

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• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
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