Fix encoded htmlspecialchars in resource overview > cache output tab

This addresses https://github.com/modxcms/revolution/issues/11833

I'm not sure if removing htmlspecialchars() from core/model/modx/processors/resource/data.class.php is unsecure, but it solves the problem.

Additionally made the textarea readonly as there is no point in it being editable. Because of this an event-blocker for the backspace key has been added, as pressing the backspace key resulted in the browser going back to the previous page in browser history.

before
![bildschirmfoto 2014-08-04 um 13 39 11](https://cloud.githubusercontent.com/assets/445501/3796899/f6238a8e-1bd0-11e4-88cd-d7e9ad68875a.png)

after
![bildschirmfoto 2014-08-04 um 14 20 00](https://cloud.githubusercontent.com/assets/445501/3796959/cc7afa40-1bd1-11e4-8ca4-d9a556c878a5.png)