Adjust z-index for Bootstrap theme to improve usage in a modal

[jelhan]

Ember Boostrap's <BsModal> and Ember Power Select do not play well together due to incompatible z-index. The dropdown content rendered by Ember Power Select is behind the modal unless consumer customizes the z-index manually. That's bad user experience. The issue is discussed here for quiet some time: ember-bootstrap/ember-bootstrap-power-select#7 (comment)

Would you accept a pull request, which changes the z-index used by Ember Power Select to be greater than Bootstrap's $zindex-modal SCSS variable? Adding .power-select-dropdown-content { z-index: $zindex-modal + 10 } to app/styles/ember-power-select/themes/bootstrap.scss and app/styles/ember-power-select/themes/bootstrap.less should be enough.