[UX] Convert the "Install from a URL" text field to a text area to allow installing multiple projects in one go.

Not sure if getting the Project Browser in core would render this page redundant, but we could convert the text field to a text area supporting multiple URLs in separate lines and we could allow uploading multiple files too (related: #440 )

If the field became a text area, we could have tutorials and how-to recipes listing the URLs of required contrib projects in an easy copy-paste list.