Add aria-label to password generator length slider, make it non-keyboard-focusable (#2478)

src/popup/generator/password-generator.component.html

@@ -178,6 +178,8 @@
               [(ngModel)]="passwordOptions.length"
               (change)="sliderChanged()"
               (input)="sliderInput()"
+              attr.aria-label="{{ 'length' | i18n }}"
+              tabindex="-1"
             />
           </div>
           <div class="box-content-row box-content-row-checkbox" appBoxRow>