Shows the open message button on focus

Fixes: CNVS-14503

Makes the open message button visible on focus.
Only focusable via the keyboard tabbing

Test-Plan:
1.) Make sure you have some messages sent to the user
	you are currently logged in as
2.) Navigate to conversation messages("Inbox")
3.) Tab through content until you are at the messages
		--Easiest way is to "skip to content"
4.) Verify that after tabbing an "open message" link
	appears

Change-Id: Ib8b7c2169e255233980a71c67831ef648858d2e3
Reviewed-on: https://gerrit.instructure.com/40198
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>
Reviewed-by: Alex Boyd <aboyd@instructure.com>
Reviewed-by: Brad Horrocks <bhorrocks@instructure.com>
Product-Review: Matt Fairbourn <mfairbourn@instructure.com>

Author: JKHiggins

app/stylesheets/pages/conversations/_conversations_new.scss

@@ -287,8 +287,20 @@
 }
 
 .open-message {
+  color: #fff;
+  background: #DC3F3F;
+  border: 1px dashed #fff;
+  font-size: 12px;
+  padding: 2px;
   left: -9999px;
   position: absolute;
+  &:focus {
+    left: 50%;
+    top: 2px;
+    width: auto;
+    height: auto;
+    z-index: 1;
+  }
 }
 
 .message-detail.span8 {