Link creation does not copy URL from selected text

[Störm]

Steps to reproduce:
User types a URL into a post or comment
User selects URL text
User clicks 'link' icon in rich text panel
Dialogue displays to create link including title (optional) and target URL

Expected:
The selected URL is copied automatically into the target URL box
User can simply press enter to create the link (if they don't want alternative text option, just to show the link and make it click-able)

Actual:
User has to re-enter URL into the URL box
Doubling the time

Most messaging and writing apps automatically convert URLs
To have to enter it twice is significant barrier to use

Tested:
This issue occurs in both mobile and desktop views
Tested on own installation and Humhub Community
The issue also occurs on https://prosemirror.net demo

Originally posted at: humhub/humhub#4949