Add prefix for tag slugs

This should be considered as a temporary fix, we may deprecate tag
slugs in the future.

Fixes #6048

Signed-off-by: Kevin Decherf <kevin@kdecherf.com>

src/Wallabag/CoreBundle/Entity/Tag.php

@@ -45,7 +45,7 @@ class Tag
 
     /**
      * @Expose
-     * @Gedmo\Slug(fields={"label"})
+     * @Gedmo\Slug(fields={"label"}, prefix="t:")
      * @ORM\Column(length=128, unique=true)
      */
     private $slug;