[Backport 12.0] Wicket development mode prevents opening documents

The original issue (CMS-10857) was caused by a javascript resource cached in the browser (DEVELOPMENT mode only). To prevent these kinds of hard-to-trace bug reports in the future, we decided to add query-string parameters containing the last-modified timestamp to Wicket resources. While query strings are a bad idea in production (they tend to be stripped off) it's fine for local development. Using this strategy has the advantage that the original file names are still recognizable, so it's easy to correlate requests with files in the code base.