Arvados

I started thinking about #20285 and then thought it was kind of annoying to have to add a config option specifically for monitoring, and it would be easier if there was a more general solution to customizing the contents of those menus.

Proposal: add a config section under Workbench with sub-sections "User", "Admin" and "Help"

Each subsection is a list of zero or more custom items which are
added to the menu.

Each item in the list has the link name and link URL. The link URL should support text substitution. Some substitution parameters:

• currently viewed UUID
• current API token
• current user UUID or username
• a property that is set on the current object?

This could also be a low-cost way to facilitate integration with other systems by giving customers an easy way to add links to other web apps that access Arvados.