Enforce default, global project and snippet visibilities

Prior to this change, it was possible for the global project and snippet
visibilities to be NULL, which could cause errors in the UI. We now
enforce a NOT NULL constraint and set the default value to private for
both visibility levels.

Closes https://gitlab.com/gitlab-org/gitlab/issues/35013

changelogs/unreleased/sh-set-admin-default-visibilities.yml

+---
+title: Enforce default, global project and snippet visibilities
+merge_request: 19188
+author:
+type: fixed

db/migrate/20191026124116_set_application_settings_default_project_and_snippet_visibility.rb

+# frozen_string_literal: true
+
+class SetApplicationSettingsDefaultProjectAndSnippetVisibility < ActiveRecord::Migration[5.2]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  def change
+    change_column_null :application_settings, :default_project_visibility, false, 0
+    change_column_default :application_settings, :default_project_visibility, from: nil, to: 0
+
+    change_column_null :application_settings, :default_snippet_visibility, false, 0
+    change_column_default :application_settings, :default_snippet_visibility, from: nil, to: 0
+  end
+end

db/schema.rb

@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema.define(version: 2019_10_26_041447) do
+ActiveRecord::Schema.define(version: 2019_10_26_124116) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "pg_trgm"
@@ -158,8 +158,8 @@ ActiveRecord::Schema.define(version: 2019_10_26_041447) do
     t.text "restricted_visibility_levels"
     t.boolean "version_check_enabled", default: true
     t.integer "max_attachment_size", default: 10, null: false
-    t.integer "default_project_visibility"
-    t.integer "default_snippet_visibility"
+    t.integer "default_project_visibility", default: 0, null: false
+    t.integer "default_snippet_visibility", default: 0, null: false
     t.text "domain_whitelist"
     t.boolean "user_oauth_applications", default: true
     t.string "after_sign_out_path"