buttons: Add support for opening external links in browser.

This commit uses the open_in_browser() function from previous
commits and allows opening external links in the browser on pressing
ENTER on the MessageLinkButton.
After this commit ZT, should be able to narrow to a message if the
link is an internal link, and open in browser if the link is an
external link. This commit does not handles internal media links in
any way currently.

Tests amended.

tests/ui_tools/test_buttons.py

@@ -449,19 +449,21 @@ def test_update_widget(
         assert isinstance(mocked_button._w, AttrMap)
 
     @pytest.mark.parametrize(
-        "link, handle_narrow_link_called",
+        "link, handle_narrow_link_called, open_in_browser_called",
         [
-            (SERVER_URL + "/#narrow/stream/1-Stream-1", True),
-            (SERVER_URL + "/user_uploads/some/path/image.png", False),
-            ("https://foo.com", False),
+            (SERVER_URL + "/#narrow/stream/1-Stream-1", True, False),
+            (SERVER_URL + "/user_uploads/some/path/image.png", False, False),
+            ("https://foo.com", False, True),
         ],
         ids=[
             "internal_narrow_link",
             "internal_media_link",
             "external_link",
         ],
     )
-    def test_handle_link(self, mocker, link, handle_narrow_link_called):
+    def test_handle_link(
+        self, mocker, link, handle_narrow_link_called, open_in_browser_called
+    ):
         self.controller.model.server_url = SERVER_URL
         self.handle_narrow_link = mocker.patch(
             BUTTONS + ".MessageLinkButton.handle_narrow_link"
@@ -471,6 +473,7 @@ def test_handle_link(self, mocker, link, handle_narrow_link_called):
         mocked_button.handle_link()
 
         assert self.handle_narrow_link.called == handle_narrow_link_called
+        assert self.controller.open_in_browser.called == open_in_browser_called
 
     @pytest.mark.parametrize(
         "stream_data, expected_response",

zulipterminal/ui_tools/buttons.py

@@ -362,6 +362,8 @@ def handle_link(self, *_: Any) -> None:
         server_url = self.model.server_url
         if self.link.startswith(urljoin(server_url, "/#narrow/")):
             self.handle_narrow_link()
+        elif not self.link.startswith(urljoin(server_url, "/user_uploads/")):
+            self.controller.open_in_browser(self.link)
 
     @staticmethod
     def _decode_stream_data(encoded_stream_data: str) -> DecodedStream: