Add a Merge Tag modifier to the Entry moderation Merge Tags to output the URL only

[rafaehlers]

There's no way at this moment to just output the URL for the entry moderation merge tags.

This will be useful for so many use cases, including generating a QRCode, as requested by a user.

Related: https://github.com/GravityKit/GravityView/blob/develop/includes/class-gravityview-entry-approval-merge-tags.php#L227

[floatytheastronaut]

FreeScout #77233.
https://support.gravitykit.com/conversation/77233?folder_id=1

[mrcasual]

Implemented in fa45dce.