Clean code: Refactor and improve code related to services

What does this MR do?

I'm reviewing, refactoring and improving the code related to services. The best way to review the changes is by reviewing each commit separately 😉

The last commit also fixes the visibility of disabled project services related to Slack because we were not listing the disabled project services but we were able to edit the individual service if we know the valid URL. This commit fixes that discrepancy between index and show/edit actions.

Eg, on GitLab.com I can see the list of Slack available services, hovering the mouse shows these two URLs:

• https://gitlab.com/arturoherrero/trainee/-/services/gitlab_slack_application/edit
• https://gitlab.com/arturoherrero/trainee/-/services/slack/edit

But I'm also able to navigate to:

• https://gitlab.com/arturoherrero/trainee/-/services/slack_slash_commands/edit

List all available services	Edit not visible service

This is part of the work related to #199388 (closed).