Convert some let to let_it_be to save re-creating DB records

The most common usage was creating a public project, so I created a
public project to be available to all spec examples.

spec/services/notification_service_spec.rb

@@ -7,6 +7,8 @@ RSpec.describe NotificationService, :mailer do
   include ExternalAuthorizationServiceHelpers
   include NotificationHelpers
 
+  let_it_be(:project, reload: true) { create(:project, :public) }
+
   let(:notification) { described_class.new }
   let(:assignee) { create(:user) }
 
@@ -513,9 +515,7 @@ RSpec.describe NotificationService, :mailer do
     end
 
     context 'confidential issue note' do
-      let(:project) { create(:project, :public) }
       let(:author) { create(:user) }
-      let(:assignee) { create(:user) }
       let(:non_member) { create(:user) }
       let(:member) { create(:user) }
       let(:guest) { create(:user) }
@@ -569,7 +569,6 @@ RSpec.describe NotificationService, :mailer do
     end
 
     context 'issue note mention', :deliver_mails_inline do
-      let(:project) { create(:project, :public) }
       let(:issue) { create(:issue, project: project, assignees: [assignee]) }
       let(:mentioned_issue) { create(:issue, assignees: issue.assignees) }
       let(:author) { create(:user) }
@@ -635,7 +634,6 @@ RSpec.describe NotificationService, :mailer do
     end
 
     context 'project snippet note', :deliver_mails_inline do
-      let!(:project) { create(:project, :public) }
       let(:snippet) { create(:project_snippet, project: project, author: create(:user)) }
       let(:author) { create(:user) }
       let(:note) { create(:note_on_project_snippet, author: author, noteable: snippet, project_id: project.id, note: '@all mentioned') }
@@ -891,11 +889,11 @@ RSpec.describe NotificationService, :mailer do
   end
 
   describe 'Participating project notification settings have priority over group and global settings if available', :deliver_mails_inline do
-    let!(:group) { create(:group) }
-    let!(:maintainer) { group.add_owner(create(:user, username: 'maintainer')).user }
-    let!(:user1) { group.add_developer(create(:user, username: 'user_with_project_and_custom_setting')).user }
+    let_it_be(:group) { create(:group) }
+    let_it_be(:maintainer) { group.add_owner(create(:user, username: 'maintainer')).user }
+    let_it_be(:user1) { group.add_developer(create(:user, username: 'user_with_project_and_custom_setting')).user }
+    let_it_be(:project) { create(:project, :public, namespace: group) }
 
-    let(:project) { create(:project, :public, namespace: group) }
     let(:issue) { create :issue, project: project, assignees: [assignee], description: '' }
 
     before do
@@ -1079,7 +1077,6 @@ RSpec.describe NotificationService, :mailer do
 
       context 'confidential issues' do
         let(:author) { create(:user) }
-        let(:assignee) { create(:user) }
         let(:non_member) { create(:user) }
         let(:member) { create(:user) }
         let(:guest) { create(:user) }
@@ -1285,7 +1282,6 @@ RSpec.describe NotificationService, :mailer do
 
       context 'confidential issues' do
         let(:author) { create(:user) }
-        let(:assignee) { create(:user) }
         let(:non_member) { create(:user) }
         let(:member) { create(:user) }
         let(:guest) { create(:user) }
@@ -1338,7 +1334,6 @@ RSpec.describe NotificationService, :mailer do
 
       context 'confidential issues' do
         let(:author) { create(:user) }
-        let(:assignee) { create(:user) }
         let(:non_member) { create(:user) }
         let(:member) { create(:user) }
         let(:guest) { create(:user) }
@@ -1390,7 +1385,6 @@ RSpec.describe NotificationService, :mailer do
 
       context 'confidential issues' do
         let(:author) { create(:user) }
-        let(:assignee) { create(:user) }
         let(:non_member) { create(:user) }
         let(:member) { create(:user) }
         let(:guest) { create(:user) }
@@ -1587,7 +1581,6 @@ RSpec.describe NotificationService, :mailer do
     let(:group) { create(:group) }
     let(:project) { create(:project, :public, :repository, namespace: group) }
     let(:another_project) { create(:project, :public, namespace: group) }
-    let(:assignee) { create(:user) }
     let(:assignees) { Array.wrap(assignee) }
     let(:author) { create(:user) }
     let(:merge_request) { create :merge_request, author: author, source_project: project, assignees: assignees, description: 'cc @participant' }
@@ -2079,8 +2072,6 @@ RSpec.describe NotificationService, :mailer do
   end
 
   describe 'Projects', :deliver_mails_inline do
-    let(:project) { create(:project) }
-
     before do
       build_team(project)
       reset_delivered_emails!
@@ -2306,7 +2297,6 @@ RSpec.describe NotificationService, :mailer do
   end
 
   describe 'ProjectMember', :deliver_mails_inline do
-    let(:project) { create(:project) }
     let(:added_user) { create(:user) }
 
     describe '#new_access_request' do
@@ -2340,7 +2330,6 @@ RSpec.describe NotificationService, :mailer do
         end
 
         it_behaves_like 'sends notification only to a maximum of ten, most recently active project maintainers' do
-          let(:project) { create(:project, :public) }
           let(:notification_trigger) { project.request_access(added_user) }
         end
       end
@@ -2470,7 +2459,6 @@ RSpec.describe NotificationService, :mailer do
     let(:private_project) { create(:project, :private) }
     let(:guest) { create(:user) }
     let(:developer) { create(:user) }
-    let(:assignee) { create(:user) }
     let(:merge_request) { create(:merge_request, source_project: private_project, assignees: [assignee]) }
     let(:merge_request1) { create(:merge_request, source_project: private_project, assignees: [assignee], description: "cc @#{guest.username}") }
     let(:note) { create(:note, noteable: merge_request, project: private_project) }
@@ -2902,7 +2890,6 @@ RSpec.describe NotificationService, :mailer do
 
   describe 'Repository cleanup', :deliver_mails_inline do
     let(:user) { create(:user) }
-    let(:project) { create(:project) }
 
     describe '#repository_cleanup_success' do
       it 'emails the specified user only' do
@@ -2933,7 +2920,6 @@ RSpec.describe NotificationService, :mailer do
 
   context 'Remote mirror notifications', :deliver_mails_inline do
     describe '#remote_mirror_update_failed' do
-      let(:project) { create(:project) }
       let(:remote_mirror) { create(:remote_mirror, project: project) }
       let(:u_blocked) { create(:user, :blocked) }
       let(:u_silence) { create_user_with_notification(:disabled, 'silent-maintainer', project) }