gs-details-page: Escape URIs for markup
When the URL contains additional parameters, like '&', these need to be escaped, otherwise gtk's markup parser claims an error.
Reproducer: Select for example kbreakout
and watch the console:
Failed to set text 'https://www.kde.org/community/donations/?app=kbreakout&source=appdata' from
markup due to error parsing markup: Error on line 1: Entity did not end with a semicolon; most
likely you used an ampersand character without intending to start an entity — escape ampersand
as &