Check hidden flag during save request

Task 'As a user I want to unhide my link (if such url is hid earlier) during Save request'
Microservice - LINKS
Subtask 'Add 'hide' link endpoint' from milestone
https://github.com/JujaLabs/juja-platform/milestones/2

1) What is this feature/fix provided for?
On save request, if the link already exists and has marked as hidden flag set to true 'true', instead of adding another copy of the link, change hidden flag value to false. 

2) List the objects you want to add or change, use appropriate API annotations
         Model  
                 Update class Link
                  @ApiModelProperty(value = "Flag means link is hidden or active, ", required = false)
              public boolean hidden 

    Service:
        Change method saveLink() logic. If link with such url is hidden - then unhide it. 

    Repository:
        Change methid getLinkByURL() if needed.

3) Are there any special requirements or constraints?

4) What behavior do you expect? Any examples?
JujaLabs / juja-platform

Check hidden flag during save request #50
