Fix EZP-23086: image thumbnail with special chars and urlalias_iri

JIRA: https://jira.ez.no/browse/EZP-23086

When using urlalias_iri transformation group urls may contain some special chars (eg ```/folder"with"quotes/```)
In this situation, the thumbnail preview on admin ui fails.

Problem: ajax / ezjscore request filters uris with htmlentities, which won't work for applying in css styles.

Fix: Revert htmlentities and re-encode uri in afftected js.

This looks hackish, but:
- It is impractical/unpretictable to modify `eZURI::transformURI()`
- Problems with using different transformation ( f.e. transformUrl will not take root/indexdir into account)