CO: Fix getImageLink for watermark module

[Shagshag]

Questions	Answers
Branch?	1.6.1.x
Description?	If watermark module is enable Link::getImageLink doesn't return a valid link for original picture.
Type?	bug fix
Category?	CO
BC breaks?	no
Deprecations?	no
Fixed ticket?
How to test?	Enable watermark, Link::getImageLink('my-picture', 12) return http://example.com/12--EZEJSDSD/my-picture.jpg instead of http://example.com/12/my-picture.jpg

[Shagshag]

As this PR was created on the GitHub interface, I will close it and recreate it with an other name.

About title convention, @mickaelandrieu rename it and remove the 'CO: ...'
so CO: or not CO: ?

[Shagshag]

Done #6206

[xBorderie]

Thanks!

About title convention, @mickaelandrieu rename it and remove the 'CO: ...'
so CO: or not CO: ?

"CO:" is only used by our changelog system, which parses the commit names.
The PR title, on the other hand, is purely for human eyes, so no need for "CO" (or others) there.
Eeasy, right? ;)