Lazily define engine routes using routes do ... end

[rmosolgo]

Fixes #5500

At least, I think it should fix that issue, but my tests fail with:

NameError (undefined local variable or method 'graphql_dashboard' for an instance of #<Class:0x000000012a0324f8>)

which is raised from the .html.erb views code. graphql_dashboard.<some_view_path> works fine in the test files, but not in the views 😖

I have been trying to trace through initializer code and RouteSet code to figure out what's missing, but I haven't been able to.