compose: Remove dividers and reduce button width at narrow mobile sizes.

At widths under 400px, buttons in the popover were running off the edge,
which is fixed by removing the dividers between them.

For the new info density, buttons in the original row, even despite
removing the dividers, were still not fitting, so their width has been
slightly reduced.

web/styles/compose.css

@@ -1154,6 +1154,12 @@ textarea.new_message_textarea {
            --compose-formatting-buttons-row-height */
         /* 28px at 20px/1em */
         height: 1.4em;
+
+        @media (width < 400px) {
+            /* Remove at mobile widths to make more space
+               for compose buttons */
+            display: none;
+        }
     }
 
     .compose_draft_button {
@@ -1231,6 +1237,14 @@ textarea.new_message_textarea {
                 display: flex;
             }
         }
+
+        .compose_control_button {
+            /* Slightly reduce width so that buttons don't overflow the available
+            space at mobile widths */
+            @media (width < 400px) {
+                width: 1.28em;
+            }
+        }
     }
 }