[Feature request] Test whether proposed filename exists. Force to change if yes.

[royjor2016]

before final submit, test if proposed filename already exists.

if yes, then display the link to that page. also prevent submit under the name to force a change.

https://commons.wikimedia.org/w/index.php?title=Commons_talk:Video2commons&oldid=746102688#[Feature_request]_test_file_name_before_confirm