Fix: Recipient tooltip not visible when details are hidden.

When message details are hidden, if user hovers on a recipient contact
name the tooltip is not visible.
Actually sometimes one can see a tiny 1-pixel horizontal line, that is
the upper border of the tooltip, hidden behind other parts of the UI.
Refactor: reordered rules in conversation.css

Rules related to message header and recipients have been placed closer
together. Rule related to menu arrow has been placed closer to option
rule.