Add mail templates for changes in MR draft status

Adds text and HTML templates for emails to be sent when a merge
request's draft/WIP status is changed.

app/views/notify/change_in_merge_request_draft_status_email.html.haml

+%p
+  #{sanitize_name(@updated_by_user.name)} changed the draft status of merge request #{@merge_request.to_reference}

app/views/notify/change_in_merge_request_draft_status_email.text.erb

+<%= "#{sanitize_name(@updated_by_user.name)} changed the draft status of merge request #{@merge_request.to_reference}" %>

spec/views/notify/change_in_merge_request_draft_status_email.html.haml_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe 'notify/change_in_merge_request_draft_status_email.html.haml' do
+  let(:user) { create(:user) }
+  let(:merge_request) { create(:merge_request) }
+
+  before do
+    assign(:updated_by_user, user)
+    assign(:merge_request, merge_request)
+  end
+
+  it 'renders the email correctly' do
+    render
+
+    expect(rendered).to have_content("#{user.name} changed the draft status of merge request #{merge_request.to_reference}")
+  end
+end

spec/views/notify/change_in_merge_request_draft_status_email.text.erb_spec.rb

+# frozen_string_literal: true
+require 'spec_helper'
+
+RSpec.describe 'notify/change_in_merge_request_draft_status_email.text.erb' do
+  let(:user) { create(:user) }
+  let(:merge_request) { create(:merge_request) }
+
+  before do
+    assign(:updated_by_user, user)
+    assign(:merge_request, merge_request)
+  end
+
+  it_behaves_like 'renders plain text email correctly'
+
+  it 'renders the email correctly' do
+    render
+
+    expect(rendered).to have_content("#{user.name} changed the draft status of merge request #{merge_request.to_reference}")
+  end
+end