Add tinymce skin files to Sprockets manifest.

We only tell sprockets to copy the compiled alchemy styles
during precompilation, but we also need the Tinymce skin
files.

Closes #3059

(cherry picked from commit 157552f)

app/assets/config/alchemy_manifest.js

@@ -3,6 +3,7 @@
 //= link tinymce/plugins/alchemy_link/plugin.min.js
 //= link tinymce/icons/remixicons/icons.js
 //= link_tree ../builds/alchemy/
+//= link_tree ../builds/tinymce/
 //= link_tree ../images/alchemy/
 //= link_tree ../../../vendor/assets/fonts/
 //= link_tree ../../../vendor/assets/images/