POC: Render the user profile tabs in Vue Router

What does this MR do and why?

Refactors the current profile_tabs.vue to a route rendered in a Vue Router. This is the ground work needed so that we can progressively move the individual tab items and move them into the sidebar menu (see #389842).

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

N/A

How to set up and validate locally

1. In rails console enable the profile_tabs_vue feature flag

   Feature.enable(:profile_tabs_vue)

2. Visit a user's profile page (http://localhost:3000/root)
3. There should be no functional difference. Both the sidebar menu and tabs should work normally.

Related to #389842 #497477 (closed)

app/controllers/users_controller.rb

@@ -28,7 +28,7 @@ class UsersController < ApplicationController
   before_action only: [:exists] do
     check_rate_limit!(:username_exists, scope: request.ip)
   end
-  before_action only: [:show, :activity, :groups, :projects, :contributed, :starred, :snippets, :followers, :following] do
+  before_action only: [:show] do
     push_frontend_feature_flag(:profile_tabs_vue, current_user)
   end