buttons: Add USER_INFO keypress events to trigger user info popup.

This commit wraps up the user-info feature by defining keypress
events to toggle user info view.

Tests added.
Fixes #511.

tests/ui_tools/test_buttons.py

@@ -328,6 +328,15 @@ def test_activate_called_once_on_keypress(
 
         assert activate.call_count == 1
 
+    @pytest.mark.parametrize("key", keys_for_command("USER_INFO"))
+    def test_keypress_USER_INFO(self, mocker, user_button, key, widget_size):
+        size = widget_size(user_button)
+        pop_up = mocker.patch("zulipterminal.core.Controller.show_user_info")
+
+        user_button.keypress(size, key)
+
+        pop_up.assert_called_once_with(user_button.user_id)
+
 
 class TestTopicButton:
     @pytest.mark.parametrize(

zulipterminal/ui_tools/buttons.py

@@ -255,6 +255,7 @@ def __init__(
         self.email = user["email"]
         self.user_id = user["user_id"]
 
+        self.controller = controller
         self._view = view  # Used in _narrow_with_compose
 
         # FIXME Is this still needed?
@@ -283,6 +284,11 @@ def _narrow_with_compose(self) -> None:
             emails=[self.email], recipient_user_ids=[self.user_id]
         )
 
+    def keypress(self, size: urwid_Size, key: str) -> Optional[str]:
+        if is_command_key("USER_INFO", key):
+            self.controller.show_user_info(self.user_id)
+        return super().keypress(size, key)
+
 
 class TopicButton(TopButton):
     def __init__(