Client's public folder renamed to static

Regular campaign sender seems to have most of the code in place. (Not tested.)

client/static/mosaico/vendor/plugins/image/index.js

@@ -0,0 +1,7 @@
+// Exports the "image" plugin for usage with module loaders
+// Usage:
+//   CommonJS:
+//     require('tinymce/plugins/image')
+//   ES2015:
+//     import 'tinymce/plugins/image'
+require('./plugin.js');