Prep Plugin for release on WordPress.org (#23)

* Update translation text domain
* Escape everything that should be escaped.
* Add nonce checks where needed.
* Sanitize all inputs.
* Apply Code style changes across the codebase.
* Correct many deprecation notices.
* Optimize load order of many filters.
* Add Proper Build script
* Use emojii flags
* Fix i18n deprecation  notice for translating too early
* Put all scripts in footer and load async

assets/css/admin.css

@@ -1851,7 +1851,7 @@ a.button.wu-button-delete.wu-border-none {
   border-width: 0;
 }
 a.button.wu-button-delete:hover {
-  border-width: 1 !important;
+  border-width: 1px !important;
   color: #8b0000;
   background-color: transparent;
 }
@@ -5193,7 +5193,7 @@ td.column-id {
   padding: 0;
   margin: -19px 0 0 -19px;
   background-image: url("../img/loader.svg");
-  background-repeat: none;
+  background-repeat: no-repeat;
   background-position: center center;
   width: 38px;
   height: 38px;