Added content listing purge functionality

This is a new button on the backend that drops the brafton_content table
and rebuilds it based on the installation script. Helpful for when
articles are deleted and the content listing row remains.
Replaced content purge with content rebuild

Content rebuild attempts to fix Joomla 2.5 component installs with the
Joomla 1.5 component backing tables by rebuilding the table and
migrating the content listing data.