Chevron icon on each message now causes a dropdown menu; hover on message for info

As does the hotkey "i". It's somewhat less appropriate because it's all actions,
but it's where our "info" menu used to be, and we can workshop a better one. "a"
feels weird to me, but maybe it's just me.

You can also hover on the .message_content to see a popover with extra message details

This is for Trac #1334.

(imported from commit f8fbf70c8502370a78159e24f3cf9589fb9d384f)

zephyr/static/js/compose.js

@@ -162,6 +162,7 @@ exports.start = function (msg_type, opts) {
         return;
     }
 
+    ui.hide_actions_dropdown();
     $("#compose_close").show();
     $("#compose_controls").hide();
     $('.message_comp').show();