FE: Don't show Design Management elements when user can't edit

What does this MR do and why?

This MR implements logic to hide Design Management toolbar and dropzone when user can't edit

Screenshots or screen recordings

With no designs

With designs

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #373295 (closed)

app/assets/javascripts/design_management/pages/index.vue

@@ -375,6 +375,7 @@ export default {
           <design-version-dropdown />
         </div>
         <div
+          v-if="canCreateDesign"
           v-show="hasDesigns"
           class="gl-display-flex gl-align-items-center"
           data-testid="design-selector-toolbar"