web: highlight Save to flash only when there are unsaved changes

The button in the header was always drawn as the primary action, on
every page, so it competed with each page's own buttons, for example
the upload button on the Firmware page. It is now a plain button and
turns primary together with the "unsaved changes" badge.

html/app.js

@@ -836,6 +836,7 @@ function isConfCmd(line){
 function setDirty(d){
   S.dirty=d;
   $("dirty").classList.toggle("show",d);
+  $("saveBtn").classList.toggle("pri",d);
 }
 function postCmd(cmd,quiet){
   return api("/cmd",{method:"POST",body:cmd}).then(function(r){