Stop reusing design component on route change

What does this MR do?

Previously, we were reusing pages/design/index.vue component when we were browsing through designs list. This MR adds a key to router-view component so _every new design will mount a component`

But why?

• this allows us to show loading spinner correctly: when we open a given design for the first time no matter if it's open from the Designs list or via pagination. Currently, when you paginate the designs it's not shown
• this allows us to accumulate all the logic connected with opening design on mounted lifecycle hook
• we don't need to call closeCommentForm on router hooks as it's always closed on the component mount

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done