classic: Show th save button only if it's usable

There are some pros and cons to this, e.g. with this change, a user
with an expired login session might be confused about why they can
no longer save. On the other hand showing the "save to tiddlyhost"
button when a save is not possible is also potentially confusing.
Anyway, let's try it this way and see how we like it.

Closely related to the changes in the previous commit for
issue #326.

rails/lib/th_file.rb

@@ -139,6 +139,7 @@ def apply_classic_mods(site_name, for_download:, is_logged_in:)
       },
 
       # Used in the ThostUploadPlugin to ensure we don't render in readonly mode
+      # and to show the 'save to tiddlyhost' button
       'TiddlyHostIsLoggedIn' => {
         text: status_is_logged_in(is_logged_in:, for_download:),
         modifier: 'TiddlyHost',

rails/tw_content/plugins/thost_upload_plugin.js.erb

@@ -200,10 +200,10 @@ if (config.shadowTiddlers.TiddlyHostIsLoggedIn == "yes") {
   // set it here to make sure TW is editable, unlike window.showBackstage
   // which is set after
   window.readOnly = false
-}
 
-// Add 'upload to tiddlyhost' button
-config.shadowTiddlers.SideBarOptions = config.shadowTiddlers.SideBarOptions
-  .replace(/(<<saveChanges>>)/,"$1<<thostUpload>>");
+  // Add the 'save to tiddlyhost' button after the regular save button
+  config.shadowTiddlers.SideBarOptions = config.shadowTiddlers.SideBarOptions
+    .replace(/(<<saveChanges>>)/,"$1<<thostUpload>>");
+}
 
 //}}}