Use toggle instead of button for enabling/disable shared runners for project settings

Problem to solve

This came from a discussion from https://gitlab.com/gitlab-org/gitlab-ce/issues/52837

This implies that shared runners are disabled. (UX: This is very confusing, and GitLab should use the toggle switch control which is much less ambiguous!)

Further details

Current implementation

Proposal

Change the button to a toggle instead to be most consistent with other on/off switches we have in the admin.

cc @dimitrieh