Improve registrar area linked user switch

- Introduce profile
- Move linked users to profile
- Use PUT

#599

spec/features/registrar/linked_users_spec.rb

@@ -0,0 +1,18 @@
+require 'rails_helper'
+
+RSpec.feature 'Registrar area linked users', settings: false do
+  given!(:current_user) { create(:api_user_with_unlimited_balance, id: 1, identity_code: 'test') }
+  given!(:linked_user) { create(:api_user_with_unlimited_balance, id: 2, identity_code: 'test',
+                                username: 'new-user-name') }
+
+  background do
+    Setting.registrar_ip_whitelist_enabled = false
+    sign_in_to_registrar_area(user: current_user)
+  end
+
+  scenario 'switches current user to a linked one' do
+    visit registrar_profile_path
+    click_link_or_button 'switch-current-user-2-btn'
+    expect(page).to have_text('You are now signed in as a user "new-user-name"')
+  end
+end