UX: Improve style of GitHub PR body in emails (#13198)

On the web, we display only an excerpt in a monospace font and the rest
of the body is hidden under ellipsis. The email displayed both of them
and it did not use the same style. This commit leaves only the excerpt
in emails and makes it use a monospace font to display it.

lib/email/styles.rb

@@ -139,8 +139,11 @@ module Email
       style('.github-info div', "display: inline; margin-right: 10px;")
       style('.github-icon-container', 'float: left;')
       style('.github-icon-container *', 'fill: #646464; width: 40px; height: 40px;')
+      style('.github-body-container', 'font-family: Consolas, Menlo, Monaco, "Lucida Console", "Liberation Mono", "DejaVu Sans Mono", "Bitstream Vera Sans Mono", "Courier New", monospace; margin-top: 1em !important;')
       style('.onebox-avatar-inline', ONEBOX_INLINE_AVATAR_STYLE)
 
+      @fragment.css('.github-body-container .excerpt').remove
+
       @fragment.css('aside.quote blockquote > p').each do |p|
         p['style'] = 'padding: 0;'
       end

spec/components/email/styles_spec.rb

@@ -185,6 +185,15 @@ describe Email::Styles do
       fragment = html_fragment('<aside class="quote"> <div class="title"> <div class="quote-controls"> <i class="fa fa-chevron-down" title="expand/collapse"></i><a href="/t/xyz/123" title="go to the quoted post" class="back"></a> </div> <img alt="" width="20" height="20" src="https://cdn-enterprise.discourse.org/boingboing/user_avatar/bbs.boingboing.net/techapj/40/54379_1.png" class="avatar">techAPJ: </div> <blockquote> <p>lorem ipsum</p> </blockquote> </aside>')
       expect(fragment.to_s.squish).to match(/^<blockquote.+<\/blockquote>$/)
     end
+
+    it "removes GitHub excerpts" do
+      stub_request(:head, "https://github.com/discourse/discourse/pull/1253").to_return(status: 200, body: "", headers: {})
+      stub_request(:get, "https://api.github.com/repos/discourse/discourse/pulls/1253").to_return(status: 200, body: onebox_response("githubpullrequest"))
+
+      onebox = Oneboxer.onebox("https://github.com/discourse/discourse/pull/1253")
+      fragment = html_fragment(onebox)
+      expect(fragment.css(".github-body-container .excerpt")).to be_empty
+    end
   end
 
   context "replace_secure_media_urls" do