Right-clicking on message should open message menu

We should change right-click behavior so that right-clicking on a message body opens the message menu. This should only happen in situations when there is no "interesting" browser menu offered, so: - No text is selected. - The click is not on a special element, like an image or link. CZO thread: [#feedback > changing right click default behavior](https://chat.zulip.org/#narrow/channel/137-feedback/topic/changing.20right.20click.20default.20behavior/with/2419760)