Allow users to see a preview of their Wikitext before final submission

[Christoph Jauera]

When submitting several files at once while having unnoticed errors in the wikitext, it can be really painful to clean the mess later on all files on Commons. It would be great to have the option of previewing the rendered wikitext before the final submission of the files.

Forwarded from the user https://en.wikipedia.org/wiki/User:DragonflySixtyseven during Technical Advice IRC Meeting.