Add `Gitlab.com?` method

To be used as a feature flag for GitLab.com-only features, such as
welcome emails.

We will be careful to only use this to disable features or functionality
that do not make sense for any installations that aren't GitLab.com. We
will not use this to restrict features from other installations or keep
them "exclusive" to GitLab.com.

lib/gitlab.rb

 require 'gitlab/git'
 module Gitlab
+  def self.com?
+    Gitlab.config.gitlab.url == 'https://gitlab.com'
+  end
 end

spec/lib/gitlab_spec.rb

+require 'rails_helper'
+
+describe Gitlab, lib: true do
+  describe '.com?' do
+    it 'is true when on GitLab.com' do
+      stub_config_setting(url: 'https://gitlab.com')
+
+      expect(described_class.com?).to eq true
+    end
+
+    it 'is false when not on GitLab.com' do
+      stub_config_setting(url: 'http://example.com')
+
+      expect(described_class.com?).to eq false
+    end
+  end
+end