title: Disable previous completed steps on mat-stepper in angular 6
body: <p>I'm working on a project in Angular 6 with Angular material and in one specific route i have a mat stepper with 6 steps configured with <code>isLinear=true</code>, so the user cant go to the next step until complete the actual step and so on.</p>
  
  <p>But when it comes to step 5, is there a button with some action, and after the user click that button, i want to prevent the user keep back and change the previously completed data.</p>
  
  <p>I'm already disabling the back button, but the user can go back to any previously completed step clicking on the step header that is showing in the top of the stepper. </p>