Link URLs only in a message's text

auto_link finds URLs and email addresses with regular expressions over
the rendered html, which can't tell an attribute value from text, so it
could put a link inside an attribute. Hand it each text node on its own,
escaped the way the serializer writes text, after the final sanitize, so
links only ever land between tags. Write the result with
preserve_newline so a code block keeps its leading blank lines.

Bump the presentation cache version so nothing rendered the old way is
served again.

Backport of 7ea05e1, 4e29c6e and 9b271d0 from #364. 1.5.2 has no
plain-text fragment cache, so only the message partial's version moves.

test/helpers/messages_helper_test.rb

@@ -25,6 +25,47 @@ class MessagesHelperTest < ActionView::TestCase
     assert_match /<strong>bold<\/strong>/, presentation
   end
 
+  test "message_presentation links URLs and email addresses in text" do
+    message = Message.create! room: rooms(:pets), body: "<div>See https://example.com/a?b=1&amp;c=2, or write to me@example.com.</div>", client_message_id: "0015", creator: users(:jason)
+
+    links = Nokogiri::HTML5.fragment(view.message_presentation(message)).css("a")
+    assert_equal [ "https://example.com/a?b=1&c=2", "mailto:me@example.com" ], links.map { it["href"] }
+    assert_equal [ "_blank", "_blank" ], links.map { it["target"] }
+    assert_equal [ "https://example.com/a?b=1&c=2", "me@example.com" ], links.map(&:text)
+  end
+
+  test "message_presentation links an email address with an apostrophe whole" do
+    message = Message.create! room: rooms(:pets), body: "<div>Write to o'brien@example.com</div>", client_message_id: "0015", creator: users(:jason)
+
+    links = Nokogiri::HTML5.fragment(view.message_presentation(message)).css("a")
+    assert_equal [ [ "mailto:o%27brien@example.com", "o'brien@example.com" ] ], links.map { [ it["href"], it.text ] }
+  end
+
+  test "message_presentation leaves URLs and email addresses in attribute values as they are" do
+    [ "class", "title" ].product([ "https://example.com", "www.example.com", "me@example.com" ]).each do |attribute, linkable|
+      value = "a> #{linkable} <em>b</em> "
+      message = Message.create! room: rooms(:pets), body: %(<div><span #{attribute}="#{value}">c</span> https://example.org</div>), client_message_id: "0015", creator: users(:jason)
+
+      presentation = Nokogiri::HTML5.fragment(view.message_presentation(message))
+      assert_equal [ value ], presentation.css("span[#{attribute}]").map { it[attribute] }, "#{attribute} with #{linkable}"
+      assert_empty presentation.css("em"), "#{attribute} with #{linkable}"
+      assert_equal [ "https://example.org" ], presentation.css("a").map { it["href"] }, "#{attribute} with #{linkable}"
+    end
+  end
+
+  test "linking text doesn't eat a code block's leading blank line" do
+    linked = view.send(:auto_link_text, "<pre>\n\n\nfirst https://example.com</pre>".html_safe)
+
+    assert_equal "\nfirst https://example.com", Nokogiri::HTML5.fragment(linked).at_css("pre").text
+  end
+
+  test "message_presentation leaves an existing link's text alone" do
+    message = Message.create! room: rooms(:pets), body: '<div><a href="https://example.com/">https://example.org/</a></div>', client_message_id: "0015", creator: users(:jason)
+
+    links = Nokogiri::HTML5.fragment(view.message_presentation(message)).css("a")
+    assert_equal [ [ "https://example.com/", "https://example.org/" ] ], links.map { [ it["href"], it.text ] }
+  end
+
   test "message_presentation shows an image's thumbnail made when it was posted" do
     presentation = view.message_presentation(attachment_message("moon.jpg", "image/jpeg", processed: true))