Rename GitlabMarkdownHelper -> MarkupHelper

It does more than solely GitLab Markdown, so rename to MarkupHelper.

app/helpers/gitlab_markdown_helper.rb → app/helpers/markup_helper.rb

 require 'nokogiri'
-module GitlabMarkdownHelper
+module MarkupHelper
   # Use this in places where you would normally use link_to(gfm(...), ...).
   #
   # It solves a problem occurring with nested links (i.e.

app/mailers/base_mailer.rb

 class BaseMailer < ActionMailer::Base
   helper ApplicationHelper
-  helper GitlabMarkdownHelper
+  helper MarkupHelper
   attr_accessor :current_user
   helper_method :current_user, :can?

spec/features/copy_as_gfm_spec.rb

 require 'spec_helper'
 describe 'Copy as GFM', feature: true, js: true do
-  include GitlabMarkdownHelper
+  include MarkupHelper
   include RepoHelpers
   include ActionView::Helpers::JavaScriptHelper

spec/features/markdown_spec.rb

@@ -26,7 +26,7 @@ require 'erb'
 describe 'GitLab Markdown', feature: true do
   include Capybara::Node::Matchers
-  include GitlabMarkdownHelper
+  include MarkupHelper
   include MarkdownMatchers
   # Sometimes it can be useful to see the parsed output of the Markdown document

spec/helpers/gitlab_markdown_helper_spec.rb → spec/helpers/markup_helper_spec.rb

 require 'spec_helper'
-describe GitlabMarkdownHelper do
+describe MarkupHelper do
   include ApplicationHelper
   let!(:project) { create(:project, :repository) }

spec/services/system_note_service_spec.rb

@@ -595,7 +595,7 @@ describe SystemNoteService, services: true do
     end
     shared_examples 'cross project mentionable' do
-      include GitlabMarkdownHelper
+      include MarkupHelper
       it 'contains cross reference to new noteable' do
         expect(subject.note).to include cross_project_reference(new_project, new_noteable)