Include link to raw job log in plain-text emails

People who prefer plain text email also might prefer the raw job log
files. So include the link directly to there.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/56849

app/views/notify/links/ci/builds/_build.text.erb

-Job #<%= build.id %> ( <%= pipeline_job_url(pipeline, build) %> )
+Job #<%= build.id %> ( <%= raw_project_job_url(pipeline.project, build) %> )

changelogs/unreleased/tc-raw-log-in-plaintext-mail.yml

+---
+title: Include link to raw job log in plain-text emails
+merge_request: 27409
+author:
+type: changed

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

+# frozen_string_literal: true
+require 'spec_helper'
+
+describe 'notify/pipeline_failed_email.text.erb' do
+  include Devise::Test::ControllerHelpers
+
+  let(:user) { create(:user) }
+  let(:project) { create(:project, :repository) }
+  let(:merge_request) { create(:merge_request, :simple, source_project: project) }
+
+  let(:pipeline) do
+    create(:ci_pipeline,
+           :failed,
+           project: project,
+           user: user,
+           ref: project.default_branch,
+           sha: project.commit.sha)
+  end
+
+  before do
+    assign(:project, project)
+    assign(:pipeline, pipeline)
+    assign(:merge_request, merge_request)
+  end
+
+  it 'renders the email correctly' do
+    job = create(:ci_build, :failed, pipeline: pipeline, project: pipeline.project)
+
+    render
+
+    expect(rendered).to have_content('Your pipeline has failed')
+    expect(rendered).to have_content(pipeline.project.name)
+    expect(rendered).to have_content(pipeline.git_commit_message.truncate(50))
+    expect(rendered).to have_content(pipeline.commit.author_name)
+    expect(rendered).to have_content("##{pipeline.id}")
+    expect(rendered).to have_content(pipeline.user.name)
+    expect(rendered).to have_content("/-/jobs/#{job.id}/raw")
+  end
+end