feat: show sharing info at bottom of AppLink's tooltip - #21197

for #19974

ShareIcon, which returns a MUI tooltip, had these problems:

1. There was no Storybook story which rendered the "Open external URL" tooltip
2. AppLink renders a button. If it has tooltip text, this button is also a Radix TooltipTrigger. Since ShareIcon is rendered as a child of the AppLink button, ShareIcon's tooltip (a nested tooltip) was never appearing. I did try turning ShareIcon into a Radix tooltip as well, but I still couldn't get the nested tooltip's text to appear, and nested tooltips are not very accessible. AFAIK there's no way to focus a child element of a button.

I've deleted the separate ShareIcon component and moved ShareIcon's tooltip text to the bottom of AppLink's tooltip:

before

^The UsersIcon on the right is a MUI tooltip trigger, but it can't receive focus. It's supposed to show the text "Shared with all authenticated users" when focused

after