compose: Align blue selection properly in Drafts.

Added a negative `outline-offset` of the same width as the grey border
for the blue selection outline around the `draft-info-box`.

This removes the gap between the blue box in the active state, while
the unselected `draft-info-box`es look the same as before, with a grey
border.

Fixes: #20950.

static/styles/drafts.css

@@ -87,6 +87,8 @@
 
         &.active {
             outline: 2px solid hsl(215, 47%, 50%);
+            /* this offset ensures no gap between the blue box and the draft in active state */
+            outline-offset: -1px;
         }
 
         .message_row {