Improve OpenAPI documentation

In the long term in would be beneficial to use the OpenAPI documentation, to generate clients for the backend-core project.

I did some experiment with client generators, and I identified some issues in the current documentation:

• The int_id schema might treated as integer enums by the client generators, so they should be replaced with regular integers.
• Operation ids should be simplified, because client methods are named after them. E.g: instructor::EvaluatorController::actionSetupEnvironment -> Instructor_Evaluator_SetupEnvironment
• The name of the module should be added to tags tags={"Runner", "Runner Auto Tester"}, thus one client could be generated for each module, instead of generating client per controllers.

Some of the changes are implemented in !128 (merged) for the new runner module, but they should be applied globally.