Console - Any keypress in hst:script property text field generates an ajax request

When editing hst:script in 7.9 console, any keystroke in the CodeMirror text area will generate an AJAX request to the backend, which causes unnecessary network traffics.

The proposed fix is as the same in CMS7-9259: replacing 'onChange' event to the 'onBlur' event of CodeMirror. By this way, ajax requests only generate when the property value text field loses focus.