Connect issues and todos links and counts in super sidebar navigation

What does this MR do and why?

• Get userCount in super_sidebar.vue
• Add href prop to super_sidebar/counter.vue to render as link
• Add label prop to super_sidebar/counter.vue to compute aria-label
• Add i18n strings for user actions
• Remove counts from mock_data.js

Screenshots or screen recordings

How to set up and validate locally

1. Enable the feature flag echo "Feature.enable(:super_sidebar_nav)" | rails c
2. Enable the "New navigation" setting toggle from the user dropdown
3. Press "Issues" and "Todos" links in super sidebar
   • Links should direct to correct location
   • Links should contain correct count value

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.

Closes #385746 (closed) #385747 (closed)