[TASK] Improve UX of Add media by url modal

When pressing ENTER in the input field, the button to process the field
is triggerd now. The OK button of the error modal is now focussed by
default.

Resolves: #70680
Releases: master
Change-Id: Ib056c5754a591e9b8b19b94cb338e75218a8bdca
Reviewed-on: http://review.typo3.org/44046


Reviewed-by: Andreas Fernandez <typo3@scripting-base.de>
Tested-by: Andreas Fernandez <typo3@scripting-base.de>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>