Do not set fixed width for thread selector dropdown box

There were cases that the fixed width for this box would cause the
thread selector to explode outside of the email header table in the
thread viewer. We should keep the select dropdown itself to the width
of the overall list of email addresses in the chain.

media/css/main.css

@@ -1247,6 +1247,11 @@ input[type=checkbox]:focus {
   margin: 2px;
 }
 
+/* For the thread viewer, keep the width inline with the parent element */
+#thread_select.custom-select {
+  width: auto;
+}
+
 th.formfieldnamecontainer {
   padding-right: 1em;
 }