Optimizations for zero config editing

* Used new EditFilterMergedContent hook instead of obsolete EditFilterMerged
* Reuse the same content object rather than reparse/revalidate it multiple times
* Automatically remove trailing comma in JSON lists
* Removed <br /> in i18n - error reporting already shows multiple errors list

Change-Id: If7312ae57798c296a431e21200675b156961823d
Merge "Optimizations for zero config editing"