Bugid 52072

This pull request addresses a defect where an unclosed references tag will hide any content following it under a puzzle piece icon in the visual editor.

This is a core defect as well, as it can be replicated on Wikipedia using the preview mode.

This is addressed in two ways:

1) Update the core Cite extension to register an error if the references tag is not closed (we will also submit this to gerrit.mediawiki.org as a pull request)
2) Register an edge case in the visual editor in cases where a parser hook tag is not closed, preventing visual mode in such scenarios

This will force the user to use the "preview" button to preview their changes, causing them to view the error message in the Cite extension, complaining that the tag was not closed. This will also prevent the loss of data, without forcing any guarantees around parsing buggy user-generated input.