Loading all sites in my sites menu sends the admin bar menu out of the viewport

[codix]

Loading all sites in my sites menu sends the admin bar menu out of the view port if it doesn't timeout before loading all. There is no point loading all sites anyway as the user can't click on links out of the view port.

For sake of usability and performance I propose we limit the number of sites added to my sites admin bar menu to 14 and add a See All which link to My Sites.

[SergeyBiryukov]

Duplicate of #15317.