Setup tabs scaffolding and feature flag

To start migrating the user tabs on the profile page we need to introduce a feature flag and setup the general scaffolding for the tabs.

Implementation plan

1. Create a user_tabs_vue feature flag and add a check to app/views/users/show.html.haml#L134
2. Create a user_tabs.vue component in app/assets/javascripts/users/components
3. Use GlTabs in user_tabs.vue
4. Setup router to change URL when changing tabs