Add post-login confirmation page for Jira app

Improves user flow when installing the GitLab for Jira
app.

app/assets/stylesheets/page_bundles/jira_connect_users.scss

+@import 'mixins_and_variables_and_functions';
+
+.jira-connect-users-container {
+  margin-left: auto;
+  margin-right: auto;
+  width: px-to-rem(350px);
+}
+
+.devise-layout-html body .navless-container {
+  @include media-breakpoint-down(xs) {
+    padding-top: 65px;
+  }
+}

app/controllers/jira_connect/users_controller.rb

+# frozen_string_literal: true
+
+class JiraConnect::UsersController < ApplicationController
+  before_action :clear_flash, only: :show
+
+  layout 'devise_experimental_onboarding_issues'
+
+  def show
+  end
+
+  private
+
+  def clear_flash
+    flash[:alert] = nil
+  end
+end

app/views/jira_connect/users/show.html.haml

+.jira-connect-users-container.gl-text-center
+  %h2= _('You are signed into GitLab.')
+
+  %p= _('You can now close this window and return to the GitLab for Jira application.')
+
+  %p= external_link _('Return to GitLab for Jira'), '#', class: 'btn btn-success'
+
+  %p= link_to _('Sign out'), '#'
+
+- add_page_specific_style 'page_bundles/jira_connect_users'

config/application.rb

@@ -182,6 +182,7 @@ module Gitlab
     config.assets.precompile << "page_bundles/ide.css"
     config.assets.precompile << "page_bundles/issues_list.css"
     config.assets.precompile << "page_bundles/jira_connect.css"
+    config.assets.precompile << "page_bundles/jira_connect_users.css"
     config.assets.precompile << "page_bundles/merge_conflicts.css"
     config.assets.precompile << "page_bundles/merge_requests.css"
     config.assets.precompile << "page_bundles/milestone.css"

config/routes/jira_connect.rb

@@ -5,6 +5,7 @@ namespace :jira_connect do
   root to: proc { [404, {}, ['']] }, as: 'base'
 
   get 'app_descriptor' => 'app_descriptor#show'
+  get :users, to: 'users#show'
 
   namespace :events do
     post 'installed'