Hide prefill-variables in Manual job UI when variables are disabled

Problem

As discussed in &16321 (comment 2353435276), when pipeline variables are disabled for a project we still display prefill-variables form in the Manual job UI.

There is no need for users to see such form if they cannot pass any variables.

Solution

Frontend checks whether project has pipeline variables enabled and only then display the variables fields. In addition, frontend could check whether the current user has permissions to pass variables, since pipeline variables can be restricted by role.

Hide display dependent on pipeline_variables_minimum_override_role