Add tooltip sentence to explain Public/Internal/Private icons

[gavin-k-lee]

The concepts of Public/Internal/Private are not immediately obvious to users:

It is great that we have alttext for them already but it would be great to have a sentence on each. E.g.

Public: Anyone on in the internet can access your project.
Internal: Anyone signed-in to {this deployment} can access your project.
Private: Only members explicitly added to this project can access it.

We can implement them as a tooltip.