chore(ckeditor): Convert blockimagepaste plugin to AMD

@jdalsem This is what I meant. Is this what you tried? Any reason not to go this way?
The AMD file would be self-contained and shouldn't result in any further fetches.
```js
define('elgg/ckeditor/blockimagepaste', function() {
  return ...;
});
require(['CKEDITOR', 'elgg/ckeditor/blockimagepaste'], function(CKEDITOR, blockimagepaste) {
  CKEDITOR.plugins.add(blockimagepaste);
})
```