Simplify runner registration token resetting

What does this MR do?

Notice: EE port at gitlab-ee!7511

This MR adds several changes related to the same topic - resetting a Runner registration token:

1. On Project settings page it adds a button for resetting the registration token and it removes the Runner token field that was confusing all GitLab users.
2. On Group settings page it adds the same button for resetting the registration token.
3. On Admin Runners settings page it adds the same button for resetting the registration token and it changes slightly the layout, as requested in #41922 (closed).
4. It refactorizes a little the partials that prints runner registration description. Thanks to this Project, Group and Admin settings of the Runner are re-using the same code to generate the button.

Admin settings

Before

After

Group settings

Before

After

Project settings

Before

After

What are the relevant issue numbers?

Closes #41922 (closed) Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/20899

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides