Add alert after Slack application is installed

Use session to only show it once after the install is
successful.

Changelog: changed
EE: true

ee/app/controllers/projects/settings/slacks_controller.rb

@@ -20,6 +20,7 @@ module Projects
           flash[:alert] = result[:message]
         end
 
+        session[:slack_install_success] = true
         redirect_to_service_page
       end
 

ee/app/views/projects/services/gitlab_slack_application/_top.html.haml

+- if session.delete(:slack_install_success)
+  = render 'shared/global_alert',
+    title: s_('SlackIntegration|GitLab for Slack was successfully installed.'),
+    variant: :success,
+    is_contained: true do
+    .gl-alert-body
+      = s_('SlackIntegration|You can now close this window and go to your Slack workspace.')

ee/spec/controllers/projects/settings/slacks_controller_spec.rb

@@ -39,6 +39,7 @@ RSpec.describe Projects::Settings::SlacksController do
         expect(response).to have_gitlab_http_status(:found)
         expect(response).to redirect_to(redirect_url(project))
         expect(flash[:alert]).to be_nil
+        expect(session[:slack_install_success]).to be(true)
       end
 
       it 'calls service and redirects with the alert if there is error' do

locale/gitlab.pot

@@ -33283,6 +33283,9 @@ msgstr ""
 msgid "SlackIntegration|GitLab for Slack"
 msgstr ""
 
+msgid "SlackIntegration|GitLab for Slack was successfully installed."
+msgstr ""
+
 msgid "SlackIntegration|Project alias"
 msgstr ""
 
@@ -33298,6 +33301,9 @@ msgstr ""
 msgid "SlackIntegration|To set up this integration press \"Add to Slack\""
 msgstr ""
 
+msgid "SlackIntegration|You can now close this window and go to your Slack workspace."
+msgstr ""
+
 msgid "SlackService|1. %{slash_command_link_start}Add a slash command%{slash_command_link_end} in your Slack team using this information:"
 msgstr ""