The button to insert an URL now opens a dialog prompting for the two
components, the URL and the description.
Any existing text selection is taken into account to pre-fill the
description field.
Closes: #6731

Co-authored-by: Otto Richter <otto@codeberg.org>
Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/7006
Reviewed-by: Otto <otto@codeberg.org>
Co-authored-by: Lucas Schwiderski <lucas@lschwiderski.de>
Co-committed-by: Lucas Schwiderski <lucas@lschwiderski.de>