Add Content Editor as an option when creating or editing a Wiki Page

Goal

• A button titled "Use new editor" is visible when the feature when the feature flag wiki_content_editor is turned on and markdown format is selected.
• Clicking on the button should show the content editor instead of the markdown editor.
• A warning should appear telling people that the content editor is in beta. A button "switch to old editor" should be a part of the warning that switches content back to the old editor.
• Switching formats should not be allowed when in the new editor. You can switch formats only when you go back to the old editor.
• When switching back to the old editor, the content in the old editor should be restored (discarding any changes in the new editor).
• When in the old editor, the format dropdown is enabled again and the user can switch between the formats again.