Database migration for caching has_confluence

Similar to other integrations that are tested for on the side nav,
(`has_external_wiki` and `has_external_issue_tracker`) this change adds
a new boolean column to `projects` to allow caching the  status of the
new confluence integration directly on the project as `has_confluence`.

https://gitlab.com/gitlab-org/gitlab/-/issues/220934

changelogs/unreleased/220934-confluence-wiki-db.yml

+---
+title: Database migration to add project_settings.has_confluence
+merge_request: 35485
+author:
+type: other

db/migrate/20200619000316_add_has_confluence_to_project_settings.rb

+# frozen_string_literal: true
+
+class AddHasConfluenceToProjectSettings < ActiveRecord::Migration[6.0]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  def up
+    with_lock_retries do
+      add_column :project_settings, :has_confluence, :boolean, default: false, null: false
+    end
+  end
+
+  def down
+    with_lock_retries do
+      remove_column :project_settings, :has_confluence
+    end
+  end
+end

db/structure.sql

@@ -14191,6 +14191,7 @@ CREATE TABLE public.project_settings (
     show_default_award_emojis boolean DEFAULT true,
     allow_merge_on_skipped_pipeline boolean,
     squash_option smallint DEFAULT 3,
+    has_confluence boolean DEFAULT false NOT NULL,
     CONSTRAINT check_bde223416c CHECK ((show_default_award_emojis IS NOT NULL))
 );
 
@@ -23517,6 +23518,7 @@ COPY "schema_migrations" (version) FROM STDIN;
 20200618105638
 20200618134223
 20200618134723
+20200619000316
 20200619154527
 20200619154528
 20200622040750

spec/lib/gitlab/import_export/safe_model_attributes.yml

@@ -705,6 +705,7 @@ ProjectCiCdSetting:
 - group_runners_enabled
 ProjectSetting:
 - allow_merge_on_skipped_pipeline
+- has_confluence
 ProtectedEnvironment:
 - id
 - project_id